Documentation/atomic_ops.txt: avoid volatile in sample code

As declaring counter as volatile is discouraged, it is best not to use it
in sample code as well.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>