Elsewere in mgr/prometheus use the 'config set' way, so update the only 'config-key set'
to make it consistent.
Signed-off-by: haoyixing <haoyixing@kuaishou.com>
By default the module will accept HTTP requests on port ``9283`` on all IPv4
and IPv6 addresses on the host. The port and listen address are both
-configurable with ``ceph config-key set``, with keys
+configurable with ``ceph config set``, with keys
``mgr/prometheus/server_addr`` and ``mgr/prometheus/server_port``. This port
is registered with Prometheus's `registry
<https://github.com/prometheus/prometheus/wiki/Default-port-allocations>`_.