docs: Make 'how to open a PR' title match filename

- This is a bit of a cheat to get around the titlecase lint. But I
  wasn't sure whether to ignore it or not. And there've been previous
  comments about making filenames match titles, so...

docs/How-To-Open-a-Homebrew-Pull-Request.md

-# How To Open a Homebrew Pull Request (and get it merged)
+# How To Open a Homebrew Pull Request
 
 The following commands are used by Homebrew contributors to set up a fork of Homebrew's Git repository on GitHub, create a new branch and create a GitHub pull request ("PR") of the changes in that branch.