Twenty Fifteen: Fix typo in a CSS comment.

props iamtakashi.
fixes #30081.

git-svn-id: https://develop.svn.wordpress.org/trunk@29995 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: Isolate :not selectors from CSS2 selectors.

props peterwilsoncc, iamtakashi.
fixes #30014.

git-svn-id: https://develop.svn.wordpress.org/trunk@29996 602fd350-edb4-49c9-b593-d223f7449a82