Fix spelling in CONTRIBUTING.md (facebook#539)

Summary:
I think there was a spelling mistake in the documentation.

Pull Request resolved: facebook#539

Test Plan: None. Just documentation.

Reviewed By: neildhar

Differential Revision: D30682309

Pulled By: Huxpro

fbshipit-source-id: 25da0a80654a9a2b64351db9696770a22d49fb05

CONTRIBUTING.md

@@ -19,7 +19,7 @@ issue first to get feedback on your idea.
 1. Fork the repo and create your branch from `main`.
 2. If you've added code that should be tested, add tests.
 3. Ensure the test suite passes and your code lints.
-4. Consider quashing your commits (`git rebase -i`). One intent alongside one
+4. Consider squashing your commits (`git rebase -i`). One intent alongside one
    commit makes it clearer for people to review and easier to understand your
    intention.
 5. If you haven't already, complete the Contributor License Agreement ("CLA").