Documentation: security: core.rst: fix warnings

Fix warnings due to missing markup, no change in content otherwise.

Signed-off-by: Daniel W. S. Almeida <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/security/keys/core.rst

···
     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,