Update comment in docker-compose.yml

The tag of the used nginx image got changed, but the comment still references the old tag.

compose-sample-2/docker-compose.yml

@@ -2,7 +2,7 @@ version: '3'
 
 services:
   proxy:
-    image: nginx:1.13 # this will use the latest version of 1.11.x
+    image: nginx:1.13 # this will use the latest version of 1.13.x
     ports:
       - '80:80' # expose 80 on host and sent to 80 in container
     volumes: