fix(flow): ignore contentHint

I don't know how to modify built-in MediaStreamTrack type, so ignore the error.

react/features/stream-effects/presenter/JitsiStreamPresenterEffect.js

 
         // Put emphasis on the text details for the presenter's stream
         // See https://www.w3.org/TR/mst-content-hint/
+        // $FlowExpectedError
         capturedStream.getVideoTracks()[0].contentHint = 'text';
 
         return capturedStream;