docs: fix metrics labels name in notification monitoring

[dacofr]

This PR fix notification documentation which use wrong labels name in argocd_notifications_deliveries_total metric
Possible labels are defined here https://github.com/argoproj/notifications-engine/blob/master/pkg/controller/metrics.go#L16

func NewMetricsRegistry(prefix string) *MetricsRegistry {
	deliveriesCounter := prometheus.NewCounterVec(
		prometheus.CounterOpts{
			Name: fmt.Sprintf("%s_notifications_deliveries_total", prefix),
			Help: "Number of delivered notifications.",
		},
		[]string{"trigger", "service", "succeeded"},
	)
     ...
}

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
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[pasha-codefresh]

Good catch, thank you