[niconico] Update fallback video title regex:

Actually this code path now is like a triple fallback and may not
be reached by anyone ever again, nevertheless...

youtube_dl/extractor/niconico.py

@@ -340,7 +340,7 @@ class NiconicoIE(InfoExtractor):
             title = self._og_search_title(webpage, default=None)
         if not title:
             title = self._html_search_regex(
-                r'<span[^>]+class="videoHeaderTitle"[^>]*>([^<]+)</span>',
+                r'<h1[^>]+class="txt-title"[^>]*>([^<]+)</h1>',
                 webpage, 'video title')
 
         watch_api_data_string = self._html_search_regex(