Don't use frame.index in docs

Author: WyattBlue

docs/index.rst

@@ -54,8 +54,8 @@ Basic Demo
 
     container = av.open(path_to_video)
 
-    for frame in container.decode(video=0):
-        frame.to_image().save('frame-%04d.jpg' % frame.index)
+    for index, frame in enumerate(container.decode(video=0)):
+        frame.to_image().save(f"frame-{index:04d}.jpg")
 
 
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