Move the example

The example under `CreateFromFile(String)` should be moved to `CreateFromFile(String, FileMode, String)`, because it actually uses that overload of the method

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* ID: 25bccff2-0202-40c0-278d-9f791e353b93
* Version Independent ID: a4d0765b-0488-c971-977c-cf4960b58b88
* Content: [MemoryMappedFile.CreateFromFile Method (System.IO.MemoryMappedFiles)](https://docs.microsoft.com/en-us/dotnet/api/system.io.memorymappedfiles.memorymappedfile.createfromfile?view=netframework-4.7.2)
* Content Source: [xml/System.IO.MemoryMappedFiles/MemoryMappedFile.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.IO.MemoryMappedFiles/MemoryMappedFile.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**

Open xml/System.IO.MemoryMappedFiles/MemoryMappedFile.xml and locate the example under CreateFromFile(String). Move it to CreateFromFile(String, FileMode, String), since the example uses that overload. Done means the example appears under the matching API overload and no longer under the single-argument method.

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