Replace the `cite` element with a `b` element to markup the comment author in comment_html5().

In HTML5 the `cite` element isn't valid for just an author's name.
"The cite element represents the title of a work [..] A person's name is not the title of a work" (http://www.w3.org/TR/html5/text-level-semantics.html#the-cite-element)

fixed #24522.

git-svn-id: https://core.svn.wordpress.org/trunk@24539 1a063a9b-81f0-0310-95a4-ce76da25c4cd