Update writesched.go comments

WriteScheduler.Push is always be called when send write request

Author: chenk008

http2/writesched.go

@@ -25,7 +25,7 @@ type WriteScheduler interface {
 	// https://tools.ietf.org/html/rfc7540#section-5.1
 	AdjustStream(streamID uint32, priority PriorityParam)
 
-	// Push queues a frame in the scheduler. In most cases, this will not be
+	// Push queues a frame in the scheduler. In most cases, this will be
 	// called with wr.StreamID()!=0 unless that stream is currently open. The one
 	// exception is RST_STREAM frames, which may be sent on idle or closed streams.
 	Push(wr FrameWriteRequest)