extractor: youtube: Set extension of AAC audio formats to m4a.

This, in particular, eases downloading both audio and videos in DASH formats
before muxing them, which alleviates the problem that I exposed on issue

Furthermore, one may argue that this is, indeed, the case for correctness's
sake.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>

youtube_dl/extractor/youtube.py

@@ -237,9 +237,9 @@ class YoutubeIE(YoutubeBaseInfoExtractor, SubtitlesInfoExtractor):
         '137': 'mp4',
         '138': 'mp4',
         '139': 'mp4',
-        '140': 'mp4',
-        '141': 'mp4',
-        '160': 'mp4',
+        '140': 'm4a',
+        '141': 'm4a',
+        '160': 'm4a',
 
         # Dash webm
         '171': 'webm',