doc/cephadm: Fix automodule generation in certmgr.rst

Wrong path was passed to automodule and no documentation was generated.
Use the right file name.

Signed-off-by: Ville Ojamo <[email protected]>
(cherry picked from commit f36746c)

Author: bluikko

doc/cephadm/certmgr.rst

@@ -90,7 +90,7 @@ Known Certificates and Keys
 The `CertMgr` class maintains a list of known certificates and keys. These are
 automatically documented below:
 
-.. automodule:: mgr.cephadm.certmgr
+.. automodule:: mgr.cephadm.cert_mgr
    :members: known_certs, known_keys
    :undoc-members:
    :show-inheritance: