Update service-clusters.md

C&P error correction

Author: L-Hudson

ee/ucp/interlock/usage/service-clusters.md

@@ -77,7 +77,7 @@ PollInterval = "3s"
     Image = "{{ page.ucp_org }}/ucp-interlock-extension:{{ page.ucp_version }}"
     Args = []
     ServiceName = "ucp-interlock-extension-us-west"
-    ProxyImage = "nginx:alpine"
+    ProxyImage = "docker/ucp-interlock-proxy:3.1.2"
     ProxyArgs = []
     ProxyServiceName = "ucp-interlock-proxy-us-west"
     ProxyConfigPath = "/etc/nginx/nginx.conf"