pubsub: fix MaxExtension doc
Fixes
@@ -197,7 +197,7 @@ type ReceiveSettings struct {
//
// The Subscription will automatically extend the ack deadline of all
// fetched Messages for the duration specified. Automatic deadline
- // extension may be disabled by specifying a duration less than 1.
+ // extension may be disabled by specifying a duration less than 0.
//
// Connections may be terminated if they last longer than 30m, which
// effectively makes that the ceiling for this value. For longer message
