add example to api docs for user index with search term

Change-Id: I767b799c291e1116e518fe461ee9df5e4f10e6a4
Reviewed-on: https://gerrit.instructure.com/43421
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brandon Broschinsky <brandonbr@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>