docs: fix example output for docker plugin ls

The output format was changed to combine tag and name in a single
column, but this change was never reflected in the docs.

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

docs/reference/commandline/plugin_push.md

@@ -29,8 +29,8 @@ The following example shows how to push a sample `user/plugin`.
 ```bash
 $ docker plugin ls
 
-ID                  NAME                  TAG                 DESCRIPTION                ENABLED
-69553ca1d456        user/plugin           latest              A sample plugin for Docker false
+ID             NAME                    DESCRIPTION                  ENABLED
+69553ca1d456   user/plugin:latest      A sample plugin for Docker   false
 
 $ docker plugin push user/plugin
 ```