Slight adjustment to centered headlines in About page. See #26387.

git-svn-id: https://develop.svn.wordpress.org/trunk@26720 602fd350-edb4-49c9-b593-d223f7449a82
Switch this header on the ABout page from title case to sentence case to match the style of all other headers on the page. See #26387.

git-svn-id: https://develop.svn.wordpress.org/trunk@26721 602fd350-edb4-49c9-b593-d223f7449a82