Doc: Fixed a few broken links and snippets

Couple of \snippet references in the ListModel docs
were pointing to files outside the exampledirs boundary.
Update those references to use the relative path.

Change-Id: Ibf9d0fda9e5280345b08cb66a4fe0db9142a42a6
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>