doc/cephfs: update cephfs-shell link
Repair the link to cephfs-shell.rst in doc/cephfs/cephfs-shell.rst that
was broken in https://github.com/ceph/ceph/pull/41165/ when
doc/cephfs/cephfs-shell.rst was moved to doc/man/8/cephfs-shell.rst.
This commit is made in response to a request by Lander Duncan that was
made on the [ceph-users] mailing list, and can be seen here: https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/F7V4CWLIYCAJ4JXI2JLNY6QPCFPR4SLA/
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit
4f8903cf80a94596a1b17b4a07affb874f21ae76)