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>