Remove broken Commmand Line guide link

[nateberkopec]

The Ruby on Rails Guides Guide recommends:

To generate all the guides, just cd into the guides directory, run bundle install and execute: rake guides:generate
It is also recommended that you work with WARNINGS=1. This detects duplicate IDs and warns about broken internal links.

Currently, on master, that produces one warning:

Generating command_line.md as command_line.html
*** BROKEN LINK: #server-with-different-backends, perhaps you meant #rails-with-databases-and-scm.

This PR removes that broken link. I have removed the entire line, because the section it is referring to has been removed (there is no discussion, as far as I can tell, in any Rails Guides about webservers other than Webrick.)