Fix incorrect comment for comment_form() in comment.module

Pretty minor bug, I guess... but it caused me to waste a half hour yesterday while dealing with another issue, since I was under the impression that the comment was right and the code was wrong, but in fact it was the other way around ;)

(Seems like what's going on here is actually a bit of a technical limitation in FAPI, though? Just because the form has errors doesn't mean the user doesn't want to hit the "Save" button after they correct them.)