mod/wiki/pagelib.php
1050: echo $OUTPUT->notification(get_string('previewwarning', 'wiki'), 'notifyproblem wiki_info');

The above line adds the class "wiki_info" to the error message. This makes the text inside darkred, and underlines it. The text inside a notifyproblem is already red, underlines generally indicate hyperlinks and there seems no reason to have this one error message in the whole of Moodle look slightly different so I suggest removing the "wiki_info" class. And since the accompanying CSS only seems to be used for this purpose, to remove those lines too (mod/wiki/styles.css lines 51 - 54)