Updated references from josegonzalez/php to Homebrew/homebrew-php.

The line in [abstract-php](https://github.com/Homebrew/homebrew-php/blob/73795319acc7a75911cda16ab69bab1ae978b746/Formula/abstract-php.rb#L459) in particular is causing problems because every output for caveats will prompt you to execute `brew --prefix josegonzalez/php/...` and that will fail if you've tapped homebrew/php, where this now officially lives.

This PR points to `brew --prefix homebrew/php/...` instead.

If backwards compatibility is needed maybe we could add a check to see which tap is used and print a message that points to the correct tap in the command.

Notes:
- The link in `Using FPM.md` does redirect properly at github.com but I considered it important for documentation.

- I left one occurrence in `Formula/abstract-php.rb` and `Formula/phpsh.rb` because they have historic importance.