Scope the Vale "terms" substitution lint to paragraphs only

- This clears up the final four title-case "Pull Request" in titles,
  which are correct.

docs/vale-styles/Homebrew/Terms.yml

@@ -3,6 +3,7 @@ extends: substitution
 message: Use '%s' instead of '%s'.
 link: 'https://github.com/Homebrew/brew/blob/master/docs/Prose-Style-Guidelines.md#terminology-words-and-word-styling'
 level: error
+scope: $paragraph
 swap:
   Pull Request: pull request
   repo: repository