Rename Prometheus alerts for consistency

Change-Id: Ic85124bc245eb80f9accd1f0882bc7e8f285defb
diff --git a/memcached/meta/prometheus.yml b/memcached/meta/prometheus.yml
index 9a5f427..b66ba01 100644
--- a/memcached/meta/prometheus.yml
+++ b/memcached/meta/prometheus.yml
@@ -2,7 +2,7 @@
 {%- if server.get('enabled', False) %}
 server:
   alert:
-    ProcstatRunningMemcached:
+    MemcachedProcessDown:
       if: >-
         procstat_running{process_name="memcached"} == 0
       {% raw %}