http2: fix RFC number typo in comment

Fix by Martin Garton in https://golang.org/cl/43370

Change-Id: I5933b136ff667c0f67ec8ebffb52d6f1235232b3
Reviewed-on: https://go-review.googlesource.com/44030
Reviewed-by: Matt Layher <mdlayher@gmail.com>

http2/writesched_priority.go

@@ -53,7 +53,7 @@ type PriorityWriteSchedulerConfig struct {
 }
 
 // NewPriorityWriteScheduler constructs a WriteScheduler that schedules
-// frames by following HTTP/2 priorities as described in RFC 7340 Section 5.3.
+// frames by following HTTP/2 priorities as described in RFC 7540 Section 5.3.
 // If cfg is nil, default options are used.
 func NewPriorityWriteScheduler(cfg *PriorityWriteSchedulerConfig) WriteScheduler {
 	if cfg == nil {