update link in api doc to request a Developer key

Change-Id: I1f3e47d253eea6a16b9fecaf8fe3e5eb5c5381a4
Reviewed-on: https://gerrit.instructure.com/43516
Product-Review: Brad Humphrey <brad@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Brad Humphrey <brad@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>