mailmap: change primary address for D. Ben Knoble

mailmap: change primary address for D. Ben Knoble

Ben uses the +github GMail trick to identify emails sent to him by folks
that found his GitHub profile. At the time, that also meant he had to
commit under the same email for GitHub to recognize his commits. He has
since found out that GitHub can be configured with more than one email
for identification, and he would prefer his canonical email to omit
mention of GitHub (where it's not relevant) going forward.

Signed-off-by: D. Ben Knoble <redacted>
Signed-off-by: Junio C Hamano <redacted>