Fix link to Pull Request help in CONTRIBUTING.md

GitHub changed their help page URLs and did not correctly redirect the
old Pull Request help URL.
Corrected terminology in comment

Really, really nitpicky change here but ... the comment was referring to the selection rule sets as "declarations", which isn't exactly the correct term. See:

http://nimbupani.com/css-vocabulary.html
http://www.impressivewebs.com/css-terms-definitions/
Merge pull request #1313 from impressivewebs/patch-1

Corrected terminology in comment
Fixed typo in `main.css`
Merge remote-tracking branch 'upstream/master'