Point CLA link in the contributing file to the correct project

[StephanBijzitter]

Summary

So I've read the Contributing Guidelines and signed the CLA, then created a pull request and found that I had not signed the CLA. Turns out it links to a different project by slackhq :-)

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [ ] I've written tests to cover the new code and functionality included in this PR.
• [x] I've read, agree to, and signed the Contributor License Agreement (CLA).