Improve phrasing of one sentence in the contributing guide

Author: pskl

guides/source/contributing_to_ruby_on_rails.md

@@ -135,7 +135,7 @@ learn about Ruby on Rails, and the API, which serves as a reference.
 You can help improve the Rails guides by making them more coherent, consistent, or readable, adding missing information, correcting factual errors, fixing typos, or bringing them up to date with the latest edge Rails.
 
 To do so, make changes to Rails guides source files (located [here](https://github.com/rails/rails/tree/master/guides/source) on GitHub). Then open a pull request to apply your
-changes to master branch.
+changes to the master branch.
 
 When working with documentation, please take into account the [API Documentation Guidelines](api_documentation_guidelines.html) and the [Ruby on Rails Guides Guidelines](ruby_on_rails_guides_guidelines.html).