Core-api: Documentation: Replace deprecated :c:func: Usage

Replace :c:func: with func() as the previous usage is deprecated.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20200810183019.22170-1-puranjay12@gmail.com


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