Improve the Contributing to Rails Guide [skip ci]
Improve the Contributing to Rails Guide [skip ci]

First commit:

This PR improves the "4.12 Commit Your Changes" section of the
Contributing to Ruby on Rails Guide:

- Fix formatting of the commit message example

- Improve readability and clarity

- Fix grammar

- Wrap lines

See the before and after images below in this PR.

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Second commit:

- Rewrite to remove the word "buffer" [skip ci]

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