Security: Documentation: Replace deprecated :c:func: Usage

Replace :c:func: with ``func()`` as the previous usage is deprecated.
Remove an extra ')' to fix broken cross reference.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Link: https://lore.kernel.org/r/20200706184956.6928-1-puranjay12@gmail.com


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