fix migration of wiki urls with anchors

test plan:
* import the package referenced in the ticket
* the link on the wiki front page pointing to
 "Header 3 on Index page (directly)" should work

fixes #CNVS-12812

Change-Id: I2d532442d49fd62efe3a333c215a5349b1629bc7
Reviewed-on: https://gerrit.instructure.com/34192
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>