docs: Alertmanager reference shows use of {{.recipient}} this isn't possible

[ak-a]

The Alertmanager implementation does not extract and pass the recipient for a notification subscription.

Reference: https://github.com/argoproj/notifications-engine/blob/master/docs/services/alertmanager.md?plain=1#L149

The pagerduty service Send() function uses dest.Recipient

In the alertmanager server Send() function there is not use of dest.Recipient (or in fact dest)