rpc: fix comment grammar (#29507)

rpc/service.go

@@ -227,7 +227,7 @@ func isSubscriptionType(t reflect.Type) bool {
 	return t == subscriptionType
 }
 
-// isPubSub tests whether the given method has as as first argument a context.Context and
+// isPubSub tests whether the given method's first argument is a context.Context and
 // returns the pair (Subscription, error).
 func isPubSub(methodType reflect.Type) bool {
 	// numIn(0) is the receiver type