docs(pubsub): update subscription retry policy defaults (googleapis#6909

)

Fixes googleapis#6903

pubsub/subscription.go

@@ -580,7 +580,7 @@ type RetryPolicy struct {
 	// given message. Value should be between 0 and 600 seconds. Defaults to 10 seconds.
 	MinimumBackoff optional.Duration
 	// MaximumBackoff is the maximum delay between consecutive deliveries of a
-	// given message. Value should be between 0 and 600 seconds. Defaults to 10 seconds.
+	// given message. Value should be between 0 and 600 seconds. Defaults to 600 seconds.
 	MaximumBackoff optional.Duration
 }