Docs: Use the term 'amend' instead of 'squash' in `CONTRIBUTING.md`

Hopefully this makes it more clear what the intended action is. For a
lot of people "squashing" means combining multiple commits into one,
which is a common practice when merging to a branch.

CONTRIBUTING.md

@@ -52,7 +52,7 @@ Nobody is perfect, and sometimes we mess things up. That said, here are some goo
   * Several categories may be combined with `+`, e.g. `LibJS+LibWeb+Browser: ...`
 * Write the commit message subject line in the imperative mood ("Foo: Change the way dates work", not "Foo: Changed the way dates work").
 * Write your commit messages in proper English, with care and punctuation.
-* Squash your commits when making revisions after a patch review.
+* Amend your existing commits when adding changes after a review, where relevant.
 * Add your personal copyright line to files when making substantive changes. (Optional but encouraged!)
 * Check the spelling of your code, comments and commit messages.
 * If you have images that go along with your code, run `optipng -strip all` on them to optimize and strip away useless metadata - this can reduce file size from multiple kilobytes to a couple hundred bytes.