Fix doc link (#3593)

Summary: Pull Request resolved: https://github.com/pytorch/audio/pull/3593

Reviewed By: nateanl

Differential Revision: D48933041

Pulled By: mthrok

fbshipit-source-id: cd05d3cf5006206ba441fdc05548bcd922ce0598

docs/source/installation.rst

@@ -30,7 +30,7 @@ Dependencies
 Optional Dependencies
 ~~~~~~~~~~~~~~~~~~~~~
 
-.. _ffmpeg:
+.. _ffmpeg_dependency:
 
 * `FFmpeg <https://ffmpeg.org>`__