Fix typo in Alertmanager configuration

Author: gotjosh

cortex/alertmanager.libsonnet

@@ -47,7 +47,7 @@
       container.withArgsMixin(
         $.util.mapToFlags($.alertmanager_args) +
         if isHA then
-          ['--cluster.listen-address=[$(POD_IP)]:%s' % $._config.alertmanager_gossip_port] +
+          ['--cluster.listen-address=[$(POD_IP)]:%s' % $._config.alertmanager.gossip_port] +
           ['--cluster.peer=%s' % peer for peer in peers]
         else [],
       ) +