Documentation/RCU: Fix minor typo

deference should actually be dereference.
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/whatisRCU.txt

@@ -237,7 +237,7 @@ rcu_dereference()
 
 	The reader uses rcu_dereference() to fetch an RCU-protected
 	pointer, which returns a value that may then be safely
-	dereferenced.  Note that rcu_deference() does not actually
+	dereferenced.  Note that rcu_dereference() does not actually
 	dereference the pointer, instead, it protects the pointer for
 	later dereferencing.  It also executes any needed memory-barrier
 	instructions for a given CPU architecture.  Currently, only Alpha