Fix self property access in video renderer

Fixes the following error:

```
error: property '_isFirstFrameRendered' not found on object of type 'FlutterRTCVideoRenderer *'; did you mean to access instance variable '_isFirstFrameRendered'?
            self._isFirstFrameRendered = false;
                ~^
```

Author: LinusU

ios/Classes/FlutterRTCVideoRenderer.m

@@ -67,7 +67,7 @@ - (void)setVideoTrack:(RTCVideoTrack *)videoTrack {
     RTCVideoTrack *oldValue = self.videoTrack;
 
     if (oldValue != videoTrack) {
-        self._isFirstFrameRendered = false;
+        _isFirstFrameRendered = false;
         if (oldValue) {
             [oldValue removeRenderer:self];
         }