MB-8347: advertise cancel XDCR endpoint without typo

We still support old call so that if somebody have hardcoded wrong
path they still have some time to migrate.

Change-Id: Ib6a21e2352f59fd18b6318da6e232726c2df1ccd
Reviewed-on: http://review.couchbase.org/26505
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>