Replace dead link to Alertmanager config reference #489

Job	Run time
editorconfig	11s
linting (lint_jsonnet)	6s
linting (lint_yaml)	9s
test (capacity-alerts)	1m 11s
golden (capacity-alerts)	1m 36s
linting (lint_adoc)	10s
test (release-4.11)	1m 54s
golden (release-4.11)	1m 25s
test (remote-write)	1m 8s
golden (remote-write)	1m 37s
test (user-workload-monitoring)	1m 30s
golden (user-workload-monitoring)	1m 34s
test (capacity-alerts-with-node-labels)	1m 10s
golden (capacity-alerts-with-node-labels)	1m 24s
test (vsphere)	1m 6s
golden (vsphere)	1m 42s
test (custom-rules)	1m 24s
golden (custom-rules)	1m 21s
test (release-4.12)	1m 15s
golden (release-4.12)	1m 25s
test (release-4.13)	1m 31s
golden (release-4.13)	1m 43s
test (team-routing)	1m 18s
golden (team-routing)	1m 35s
	29m 25s

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