http2: standardize RFC mention format

Updates golang/go#15258

Change-Id: Ieaea9564a6e300938a75d3382151c46e9bde1e59
Reviewed-on: https://go-review.googlesource.com/21903
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/http2.go

@@ -322,7 +322,7 @@ func mustUint31(v int32) uint32 {
 }
 
 // bodyAllowedForStatus reports whether a given response status code
-// permits a body. See RFC2616, section 4.4.
+// permits a body. See RFC 2616, section 4.4.
 func bodyAllowedForStatus(status int) bool {
 	switch {
 	case status >= 100 && status <= 199: