Fix pull by digest example

Also reduced the column spacing in the output, so
that more output is visible in the documentation.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/userguide/dockerimages.md

@@ -504,13 +504,13 @@ unchanged, the digest value is predictable. To list image digest values, use
 the `--digests` flag:
 
     $ docker images --digests | head
-    REPOSITORY                         TAG                 DIGEST                                                                     IMAGE ID            CREATED             VIRTUAL SIZE
-    ouruser/sinatra                    latest              sha256:cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf    5db5f8471261        11 hours ago        446.7 MB
+    REPOSITORY        TAG      DIGEST                                                                     IMAGE ID      CREATED       VIRTUAL SIZE
+    ouruser/sinatra   latest   sha256:cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf    5db5f8471261  11 hours ago  446.7 MB
 
 When pushing or pulling to a 2.0 registry, the `push` or `pull` command
 output includes the image digest. You can `pull` using a digest value.
 
-    $ docker pull ouruser/sinatra@cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf
+    $ docker pull ouruser/sinatra@sha256:cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf
 
 You can also reference by digest in `create`, `run`, and `rmi` commands, as well as the
 `FROM` image reference in a Dockerfile.