documentation: Fix variable-name typo in memory-barriers.txt

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 13feb69..3d06f98 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -746,7 +746,7 @@
 evaluation.  Consider this example:
 
 	q = READ_ONCE_CTRL(a);
-	if (a || 1 > 0)
+	if (q || 1 > 0)
 		ACCESS_ONCE(b) = 1;
 
 Because the first condition cannot fault and the second condition is