Replace call to Show Source button to view file on GitHub

There is no "Show Source" link in the documentation how to page,
but you can still see the raw rST bare formatting if you jump
over to the relevant page hosted on the GitHub repo.

Also:

- Plugin howto: Put command line entry on own line, easier to read
- FAQ - How many users: Write out number when under 10
Merge pull request #1601 from randomecho/docs-contribute

Replace call to Show Source button to view file on GitHub [ci skip]