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.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>