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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/atomic_ops.txt

···
 C integer type will fail.  Something like the following should
 suffice:
 
-	typedef struct { volatile int counter; } atomic_t;
+	typedef struct { int counter; } atomic_t;
 
 Historically, counter has been declared volatile.  This is now discouraged.
 See Documentation/volatile-considered-harmful.txt for the complete rationale.