remove &nbsp; from notification preference tooltip

fixes CNVS-14179

note that the &nbsp; in the submission comment tooltip
is being removed by a different commit

test plan
- view 'Grading' tooltip in notification preferences
- ensure that '&nbsp;' does not appear

Change-Id: Id6c571b2f6abe170a4249c729f3af5a91a8b0ba5
Reviewed-on: https://gerrit.instructure.com/37580
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>