Corrected grammar errors in README.

[smalakys]

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• I've a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferably).
• I've written tests to cover the new code and functionality included in this PR.
• I've read, agree to, and signed the Contributor License Agreement (CLA).

PR Summary

Corrected grammar errors in README.

Related Issues

Does not apply.

Test strategy

Does not apply.

[coveralls]

Coverage remained the same at 29.268% when pulling 0933c1f on smalakys:master into f9f2fd5 on slackhq:master.

[coveralls]

Coverage remained the same at 29.268% when pulling 0933c1f on smalakys:master into f9f2fd5 on slackhq:master.

[Roach]

Thanks for fixing these! If you sign the CLA (linked above), I'll merge these tonight 👍

[smalakys]

I had already signed the CLA prior to submitting this PR. I don't know why it doesn't show up, so I signed it again just now.

[Roach]

Merging this now, thanks @smalakys 🎉