doc/cephadm: fix spec webhook key name in services/monitoring.rst

a8498ddad536305cdeda4699932dc4de7a93f540 changed the config key in spec
file example but missed the same key name in the following text.

Fixes: https://tracker.ceph.com/issues/79139
Signed-off-by: Ville Ojamo <git2233+ceph@ojamo.eu>