edited usage to show variables name

more clear usage for one-time override feature

deploy.sh

@@ -26,8 +26,8 @@ if [ $# -lt 2 ]; then
   echo "Usage: $0 <service> <service name> [branch/tag]"
   echo
   echo "Current settings:"
-  echo "- Source      · ${DEPLOY_SOURCE}"
-  echo "- Destination · ${DEPLOY_DESTINATION}"
+  echo "- \$DEPLOY_SOURCE      · ${DEPLOY_SOURCE}"
+  echo "- \$DEPLOY_DESTINATION · ${DEPLOY_DESTINATION}"
   exit
 fi