[youtube] Fix acodec and vcodec order

In RFC6381, there's no rule stating that the first part of codecs should
be video and the second part should be audio, while it seems the case
for data reported by YouTube.

youtube_dl/extractor/youtube.py

@@ -1487,7 +1487,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
                             if codecs:
                                 codecs = codecs.split(',')
                                 if len(codecs) == 2:
-                                    acodec, vcodec = codecs[0], codecs[1]
+                                    acodec, vcodec = codecs[1], codecs[0]
                                 else:
                                     acodec, vcodec = (codecs[0], 'none') if kind == 'audio' else ('none', codecs[0])
                                 dct.update({