fixed code example in Compose startup order per feedback (#3275)

Signed-off-by: Victoria Bialas <victoria.bialas@docker.com>

compose/startup-order.md

@@ -71,8 +71,11 @@ script:
         >&2 echo "Postgres is up - executing command"
         exec $cmd
 
-    You can use this as a wrapper script as in the previous example, by setting
-    `command: ["./wait-for-postgres.sh", "db", "python", "app.py"]`.
+    You can use this as a wrapper script as in the previous example, by setting:
+
+    ```none
+    command: ["./wait-for-postgres.sh", "db", "python", "app.py"]
+    ```
 
 
 ## Compose documentation