Reformat README of extractors module

PiperOrigin-RevId: 377031562

library/extractor/README.md

-# ExoPlayer extractor library module #
+# ExoPlayer extractor library module
 
 Provides media container extractors.
 
-## Links ##
+## Links
 
-* [Javadoc][]: Classes matching `com.google.android.exoplayer2.extractor.*`
-  belong to this module.
+*   [Javadoc][]: Classes matching `com.google.android.exoplayer2.extractor.*`
+    belong to this module.
 
 [Javadoc]: https://exoplayer.dev/doc/reference/index.html
-