doc: Repair some whitespace damage

A diagram in whatisRCU.txt has space character before tabs.  This commit
therefore makes this diagram consistent with elsewhere in the document:
Use one leading tab, followed by spaces for any additional whitespace
required.
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

Documentation/RCU/whatisRCU.txt

@@ -319,7 +319,7 @@ reader, updater, and reclaimer.
 	    |                           |              | rcu_read_unlock()
 	    |        rcu_dereference()  |              |
 	    +---------+                 |              |
-       | updater |<---------------------+              |
+	    | updater |<----------------+              |
 	    +---------+                                V
 	    |                                    +-----------+
 	    +----------------------------------->| reclaimer |