Fix fallback for the welcome-tour video

When running gnome-welcome-tour in Japanese locale, we would
find that /usr/share/help/ja/gnome-help/ exists, and then proceed
to show the nonexisting
/usr/share/help/ja/gnome-help/figures/gnome-yelp-intro.webm
video. Instead of checking for the directory, check for the existence
of the file we're actually interested in. Fixes
https://bugzilla.redhat.com/show_bug.cgi?id=948683