Update user_playlists_contents.py

results was never used in the show_tracks method.

examples/user_playlists_contents.py

@@ -6,7 +6,7 @@ import spotipy
 import spotipy.util as util
 
 def show_tracks(results):
-    for i, item in enumerate(tracks['items']):
+    for i, item in enumerate(results['items']):
         track = item['track']
         print("   %d %32.32s %s" % (i, track['artists'][0]['name'], track['name']))