RESTART_ON_UPGRADE incorrectly named ES_RESTART_ON_UPGRADE in sysconfig (elastic#19976)

Closes elastic#19950

Author: clintongormley

distribution/src/main/packaging/env/elasticsearch

@@ -24,7 +24,7 @@
 #ES_JAVA_OPTS=
 
 # Configure restart on package upgrade (true, every other setting will lead to not restarting)
-#ES_RESTART_ON_UPGRADE=true
+#RESTART_ON_UPGRADE=true
 
 ################################
 # Elasticsearch service