Should clean up URDF example image, upstream to ROS wiki

Migrating from Asana task on 2019-04-01
https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdf_tutorials/pull-requests/5/page/1#comment-97161755

> Eric Cousineau
> The frames on this image look very… imprecise… I will try to touch them up, and possibly suggest an update on the URDF page.
> ...
> Actually, the frames that we have for SDF look much better.
> Is there a way that we could make cleanup the URDF version to look similar (as far having lines directly aligned), and submit that change to the URDF doc wiki?
>

Regarding image:
http://wiki.ros.org/urdf/XML/model