docs: security: core.rst: Fix several warnings

Multi-line literal markups only work when they're idented at the
same level, with is not the case here:

   Documentation/security/keys/core.rst:1597: WARNING: Inline literal start-string without end-string.
   Documentation/security/keys/core.rst:1597: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1597: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1598: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1598: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1600: WARNING: Inline literal start-string without end-string.
   Documentation/security/keys/core.rst:1600: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1600: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1600: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1600: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1666: WARNING: Inline literal start-string without end-string.
   Documentation/security/keys/core.rst:1666: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1666: WARNING: Inline emphasis start-string without end-string.
   Documentation/security/keys/core.rst:1666: WARNING: Inline emphasis start-string without end-string.

Fix it by using a code-block instead.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>