Wrong install instructions for Debian machines - npm package missing

There's a minor bug in documentation here:

https://keybase.io/docs/command_line/prerequisites

At least on Debian machines (can't really speak for Ubuntu, Mint and other derivatives), npm is in it's own package npm, which unlike nodejs-legacy requests the whole nodejs structure as well.

So this:
`apt-get install nodejs-legacy`
should be replaced by this:
`apt-get install nodejs-legacy npm`

Open the prerequisites page linked in the issue and find the Debian installation command containing `apt-get install nodejs-legacy`. Update it to include the `npm` package, then verify that the displayed instructions match the requested command.

Written by the indexing model from the issue text.