Fix typo

PiperOrigin-RevId: 343438203

RELEASENOTES.md

@@ -80,7 +80,7 @@
         that lie outside the length of the cue text.
 *   Metadata retriever:
     *   Parse Google Photos HEIC motion photos metadata.
-*   Media2 extention:
+*   Media2 extension:
     *   Notify onBufferingEnded when the state of origin player becomes
         STATE_IDLE or STATE_ENDED.
     *   Allow to remove all playlist items that makes the player reset.