Update to correct Homebrew formula name

The current readme lists the formula name as `pt` when it's actually `the_platinum_searcher`<sup>[1](https://github.com/Homebrew/homebrew/blob/master/Library/Formula/the_platinum_searcher.rb)</sup>.

1 - https://github.com/Homebrew/homebrew/blob/master/Library/Formula/the_platinum_searcher.rb