bring back short_name example in Users API documentation

this appears to have been inadvertently lost during the
@object -> @model transition

test plan:
 - look at Users API documentation
 - the example user should have a short name

Change-Id: Ia85817de291806665d912ddc14953f8f049c2357
Reviewed-on: https://gerrit.instructure.com/31151
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>