Update ubuntu.md

Instruction did not mention having to start the docker service before testing 'hello world'. Following the instruction literally leads to confusing errors.

content/manuals/engine/install/ubuntu.md

@@ -176,6 +176,7 @@ Docker from the repository.
 3. Verify that the installation is successful by running the `hello-world` image:
 
    ```console
+   $ sudo service docker start
    $ sudo docker run hello-world
    ```