doc/cephadm: remove downgrade reference from upgrade docs

This has been in here for years, but cephadm will block
attempted upgrades to lower versions and we generally
don't want people to think this is supported or safe.

Signed-off-by: Adam King <[email protected]>

Author: adk3798

doc/cephadm/upgrade.rst

@@ -56,13 +56,13 @@ Before you use cephadm to upgrade Ceph, verify that all hosts are currently onli
 
    ceph -s
 
-To upgrade (or downgrade) to a specific release, run the following command:
+To upgrade to a specific release, run a command of the following form:
 
 .. prompt:: bash #
 
   ceph orch upgrade start --ceph-version <version>
 
-For example, to upgrade to v16.2.6, run the following command:
+For example, to upgrade to v16.2.6, run a command of the following form:
 
 .. prompt:: bash #