Fix some grammar mistakes in CONTRIBUTING.md

Created by: mrseanbaines

Reading through the contributing documentation, I noticed a few small grammar mistakes and inconsistencies.

Created by: facebook-github-bot

Hi mrseanbaines! Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file.In order for us to review and merge your code, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

If you have received this in error or have any questions, please contact us at cla@fb.com. Thanks!

added CLA Signed label

Created by: facebook-github-bot

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

added tag: documentation label

closed