switch content migration select icon

fixes #CNVS-13640

Change-Id: If65ae0e3c49a2336cd0c1a47fc52721f6397e43e
Reviewed-on: https://gerrit.instructure.com/36589
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>