Fix duplicate word "prevents" (#922)

Author: JonnyBurger

index.src.html

@@ -6217,7 +6217,7 @@
         [=active=] {{AudioEncoder}} or {{VideoEncoder}} in the same
         [=/global object=].
 
-        NOTE: This prevents prevents breaking long running transcoding tasks.
+        NOTE: This prevents breaking long running transcoding tasks.
 
     - An {{AudioDecoder}}, when its tab is audibly playing audio.