Fix Markdown formatting of experimental "stacks"

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit c5301ca35d)
Signed-off-by: Andrew Hsu <andrewhsu@docker.com>

components/cli/experimental/docker-stacks-and-bundles.md

@@ -161,21 +161,18 @@ A service has the following fields:
         only specify the container port to be exposed. These ports can be
         mapped on runtime hosts at the operator's discretion.
     </dd>
-
     <dt>
         WorkingDir <code>string</code>
     </dt>
     <dd>
         Working directory inside the service containers.
     </dd>
-
     <dt>
         User <code>string</code>
     </dt>
     <dd>
         Username or UID (format: <code>&lt;name|uid&gt;[:&lt;group|gid&gt;]</code>).
     </dd>
-
     <dt>
         Networks <code>[]string</code>
     </dt>