Documentation: kernel-hacking: change 'current()' to 'current'

Change 'current()' heading to 'current' to reflect usage.
Signed-off-by: Joe Pater <02joepater06@gmail.com>
Link: https://lore.kernel.org/r/20210111103240.7445-1-02joepater06@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-hacking/hacking.rst

@@ -346,8 +346,8 @@ routine.
 Before inventing your own cache of often-used objects consider using a
 slab cache in ``include/linux/slab.h``
 
-:c:func:`current()`
--------------------
+:c:macro:`current`
+------------------
 
 Defined in ``include/asm/current.h``