doc/mgr: Add root CA cert instructions to rgw.rst

doc/mgr: Add root CA cert instructions to rgw.rst

Add documentation for adding fs_id in root_ca_cert upgrade path

Fixes: https://tracker.ceph.com/issues/70014
Signed-off-by: Anuradha Gadge <anuradha.gadge@ibm.com>
(cherry picked from commit 76106dd9890fdbd9440a5f7de85e9d5de6d0a2b4)

doc/mgr: edit grammar and formatting of rgw.rst

Improve the grammar and correct the formatting of the "Upgrading root ca
certificates" procedure that was added to the documentation in https://github.com/ceph/ceph/pull/61867

Fixes: https://tracker.ceph.com/issues/70014
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 7d9298e3de74e91db116c79a7087f559464ae52d)