added missing depends_on, corrected indentation

...to match what is in compose file

Signed-off-by: Charlie Arehart <charlie@carehart.org>

nginx-golang/README.md

@@ -21,7 +21,9 @@ services:
   frontend:
     build: frontend
     ports:
-    - 80:80
+      - 80:80
+    depends_on:
+      - backend
   backend:
     build: backend
 ```