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

Documentation/security/credentials.rst

@@ -453,9 +453,9 @@ still at this point.
 
 When replacing the group list, the new list must be sorted before it
 is added to the credential, as a binary search is used to test for
-membership.  In practice, this means :c:func:`groups_sort` should be
-called before :c:func:`set_groups` or :c:func:`set_current_groups`.
-:c:func:`groups_sort)` must not be called on a ``struct group_list`` which
+membership.  In practice, this means groups_sort() should be
+called before set_groups() or set_current_groups().
+groups_sort() must not be called on a ``struct group_list`` which
 is shared as it may permute elements as part of the sorting process
 even if the array is already sorted.