Update 'up-for-grabs' link in CONSTRIBUTING.md (#2280)
Update 'up-for-grabs' link in CONSTRIBUTING.md (#2280)

The previous link redirected to https://github.com/octokit/octokit.net/issues?q=label%3Aup-for-grabs, which would also show closed issues; the new link only shows open issues.
octokit/octokit.net,octokit/octokit.net