Fix typo in f5840a

When filtering SSRCes we were running the filter operation on the source
slice and not the filtered slice. Causing us to ignore all the filter
operations that had been previously run.

Author: Sean-Der

peerconnection.go

@@ -1235,7 +1235,7 @@ func (pc *PeerConnection) startRTPReceivers(incomingTracks []trackDetails, curre
 				for _, track := range receiver.Tracks() {
 					for _, ssrc := range incomingTrack.ssrcs {
 						if ssrc == track.SSRC() {
-							filteredTracks = filterTrackWithSSRC(incomingTracks, track.SSRC())
+							filteredTracks = filterTrackWithSSRC(filteredTracks, track.SSRC())
 						}
 					}
 				}