DONTBUILD because this is a comment-only change and hence doesn't affect
behavior at all.
I ran across this typo in nsPageFrame.cpp, and figured I might as well grep for
it and fix it more comprehensively. And while I'm at it, I also fixed some
neighboring instances of a duplicated word around a linebreak, in
nsXMLContentSerializer.h.
Differential Revision: https://phabricator.services.mozilla.com/D100852