touch in PowerShell script

This script: https://githubtraining.github.io/training-manual/#/18_create_local_repo?id=powershell uses the `touch` command which PowerShell doesn't recognise.

````
for ($d=1; $d -le 6; $d++) { touch file$d.md; git add file$d.md; git commit -m "adding file$d.md"; }
>>
touch : The term 'touch' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included,
verify that the path is correct and try again.
At line:1 char:30
+ for ($d=1; $d -le 6; $d++) { touch file$d.md; git add file$d.md; git ...
+ ~~~~~
+ CategoryInfo : ObjectNotFound: (touch:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

fatal: pathspec 'file1.md' did not match any files
On branch master
nothing to commit, working tree clean
touch : The term 'touch' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included,
verify that the path is correct and try again.
At line:1 char:30
+ for ($d=1; $d -le 6; $d++) { touch file$d.md; git add file$d.md; git ...
+ ~~~~~
+ CategoryInfo : ObjectNotFound: (touch:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

fatal: pathspec 'file2.md' did not match any files
On branch master
nothing to commit, working tree clean
touch : The term 'touch' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included,
verify that the path is correct and try again.
At line:1 char:30
+ for ($d=1; $d -le 6; $d++) { touch file$d.md; git add file$d.md; git ...
+ ~~~~~
+ CategoryInfo : ObjectNotFound: (touch:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

fatal: pathspec 'file3.md' did not match any files
On branch master
nothing to commit, working tree clean
touch : The term 'touch' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included,
verify that the path is correct and try again.
At line:1 char:30
+ for ($d=1; $d -le 6; $d++) { touch file$d.md; git add file$d.md; git ...
+ ~~~~~
+ CategoryInfo : ObjectNotFound: (touch:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

fatal: pathspec 'file4.md' did not match any files
On branch master
nothing to commit, working tree clean
touch : The term 'touch' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included,
verify that the path is correct and try again.
At line:1 char:30
+ for ($d=1; $d -le 6; $d++) { touch file$d.md; git add file$d.md; git ...
+ ~~~~~
+ CategoryInfo : ObjectNotFound: (touch:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

fatal: pathspec 'file5.md' did not match any files
On branch master
nothing to commit, working tree clean
touch : The term 'touch' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included,
verify that the path is correct and try again.
At line:1 char:30
+ for ($d=1; $d -le 6; $d++) { touch file$d.md; git add file$d.md; git ...
+ ~~~~~
+ CategoryInfo : ObjectNotFound: (touch:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

fatal: pathspec 'file6.md' did not match any files
On branch master
nothing to commit, working tree clean

Apri la sezione PowerShell all’URL collegato del manuale di formazione e ispeziona il ciclo che crea file1.md fino a file6.md prima di aggiungerli ed eseguire il commit con Git. Conferma che il comando documentato funziona in PowerShell e che tutti e sei i file vengono creati, aggiunti e sottoposti a commit senza gli errori segnalati.

Scritto dal modello di indicizzazione a partire dal testo della issue.