Add cargo fmt to CONTRIBUTING.md

[arctic-alpaca]

The CI uses cargo +nightly fmt --all -- --check to check code formatting but that's not documented in the "Test before you commit" section of CONTRIBUTING.md.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
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[JohnTitor]

Hey, thanks for the PR! I've submitted #3458 to cover more things, closing in favor of that.