mw.ViewPageTarget: Be more careful when updating page title

If there is more than one span in #firstHeading, we only want to
update the first one.

This will still break if something inserts elements *before* the title
span, but fixing that would take more thought (we probably should
rebuild the entire heading element and fire 'wikipage.content' on it).

Bug: 69857
Change-Id: Id78b9b8275a57c9b3f3f1dbd0aaca356f94d0f03