Twenty Fifteen: Fix typo in a CSS comment.

props iamtakashi.
fixes #30081.
Built from https://develop.svn.wordpress.org/trunk@29995


git-svn-id: http://core.svn.wordpress.org/trunk@29736 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: Isolate :not selectors from CSS2 selectors.

props peterwilsoncc, iamtakashi.
fixes #30014.
Built from https://develop.svn.wordpress.org/trunk@29996


git-svn-id: http://core.svn.wordpress.org/trunk@29737 1a063a9b-81f0-0310-95a4-ce76da25c4cd