doc/rados: fix link in common.rst

doc/rados: fix link in common.rst

Fix a link in doc/rados/configuration/common.rst that was missing its
final letter, causing a 404 error when readers attempted to follow it.

This bug was reported by stalwart friend of the Ceph documentation
project Eugen Block, who is here credited as a co-author. This bug was
reported at https://pad.ceph.com/p/Report_Documentation_Bugs.

Co-authored-by: Eugen Block <eblock@nde.ag>
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 8128ce20fef3d75c6251691d10af21861c01e155)