Update type hint for CreditsAction::getAuthor()

This function calls $page->getUserText() and $page->getTimestamp(). These
functions are not specified in the interface Page (which actually specifies
nothing at all). Page is only implemented by WikiPage and Article. Article
does not implement getUserText() and getTimestamp(). WikiPage does.
CreditsAction::getAuthor() cannot ever be given an Article, because that
would cause a fatal error "call to undefined method".

Change-Id: I0ba29622b7307845345ce645cb63b53614aaf2ab