docs: atomic_bitops.txt: add a title for this document

This document misses a title. Add it, in order to follow
the documentation standard.

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

Documentation/atomic_bitops.txt

@@ -1,6 +1,6 @@
-
-On atomic bitops.
-
+=============
+Atomic bitops
+=============
 
 While our bitmap_{}() functions are non-atomic, we have a number of operations
 operating on single bits in a bitmap that are atomic.