[Contributing] Clarify the rebase operation

I just mentioned the fact that `push -f` is usually required when rebasing and explicit that the "2.2 when bugfixing" is a follow up on the previous example (Not necessarily explicit at first).
[Contributing] Fix single vs double backtick issues
[Contributing] Add PSR-1 and 2 links
[Contributing] Replace mentions of 2.2 for 2.3

As per WouterJ request: https://github.com/symfony/symfony-docs/pull/3421#issuecomment-31585092
[Contributing] Fix "When you finish" => "When you are finished"

As per @xabbuh suggestion: https://github.com/symfony/symfony-docs/pull/3421#discussion_r8661716
[Contributing] Replace HEAD~3 by upstream/master

As per @cordoval suggestion: https://github.com/symfony/symfony-docs/pull/3421#discussion_r8670868