Fix link for README.rst (#4653)

Co-authored-by: Vasilis Vryniotis <[email protected]>

Author: zhiqwang

README.rst

@@ -112,7 +112,9 @@ Video Backend
 =============
 Torchvision currently supports the following video backends:
 
-* [pyav](https://github.com/PyAV-Org/PyAV) (default) - Pythonic binding for ffmpeg libraries. 
+* `pyav`_ (default) - Pythonic binding for ffmpeg libraries.
+
+.. _pyav : https://github.com/PyAV-Org/PyAV
 
 * video_reader - This needs ffmpeg to be installed and torchvision to be built from source. There shouldn't be any conflicting version of ffmpeg installed. Currently, this is only supported on Linux.