Helm - Prometheus: Change example alerting endpoint port to http

The Helm Prometheus chart expects alertingEndpoints to have a port
of http, not a port number.
I'm unsure of the reasoning behind this, but this corrects the
documentation to reflect the current behaviour.

See #1356

helm/prometheus/Chart.yaml

@@ -7,4 +7,4 @@ maintainers:
 name: prometheus
 sources:
   - https://github.com/coreos/prometheus-operator
-version: 0.0.40
+version: 0.0.41

helm/prometheus/values.yaml

@@ -4,7 +4,7 @@
 alertingEndpoints: []
 #   - name: ""
 #     namespace: ""
-#     port: 9093
+#     port: http
 #     scheme: http
 
 ## Prometheus configuration directives