Fix production-cluster-with-metrics.yaml (#717)

Fixed a missing parenthesis

Author: edgarlanting

examples/production-cluster-with-metrics.yaml

@@ -13,5 +13,5 @@ spec:
     prometheus.io/port: '9101'
     prometheus.io/scrape_interval: '5s'
   mode: Cluster
-  image: arangodb/arangodb:3.7.10'
+  image: 'arangodb/arangodb:3.7.10'
   environment: Production