README: the_platinum_searcher is now in Homebrew

``the_platinum_searcher`` has now been added to the main Homebrew
repository.

For more info, refer to the commit:
https://github.com/Homebrew/homebrew/commit/0641864c6f4f2495344ed0ab1762085badc79dc6
Merge pull request #46 from dplarson/readme-homebrew-official

README: the_platinum_searcher is now in Homebrew