Fixed a typo in CONTRIBUTING.md

[ArtBears]

Small change to the CONTRIBUTING.md file.

r? @steveklabnik

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[steveklabnik]

@bors: r+ rollup

thank you!

[bors]

📌 Commit 9a6a72b has been approved by steveklabnik

[bors]

⌛ Testing commit 9a6a72b with merge cfebdea...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: steveklabnik
Pushing cfebdea to master...