transport: fix minor typo in 'GoAway' godoc (#1284)

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

transport/transport.go

@@ -534,7 +534,7 @@ type ClientTransport interface {
 	// once the transport is initiated.
 	Error() <-chan struct{}
 
-	// GoAway returns a channel that is closed when ClientTranspor
+	// GoAway returns a channel that is closed when ClientTransport
 	// receives the draining signal from the server (e.g., GOAWAY frame in
 	// HTTP/2).
 	GoAway() <-chan struct{}