Documentation: security: core.rst: fix warnings

Fix warnings due to missing markup, no change in content otherwise.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20191122041806.68650-1-dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/security/keys/core.rst

@@ -1102,7 +1102,7 @@ payload contents" for more information.
     See also Documentation/security/keys/request-key.rst.
 
 
- *  To search for a key in a specific domain, call:
+ *  To search for a key in a specific domain, call::
 
 	struct key *request_key_tag(const struct key_type *type,
 				    const char *description,