rcu: Make documentation give more realistic rcutorture duration

The torture.txt documentation gives an example rcutorture run with a
100-second duration.  This is ridiculously short, unless maybe testing
a fix for a egregious bug.  Use a more-realistic one-hour duration for
the example.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/torture.txt

@@ -278,7 +278,7 @@ The following script may be used to torture RCU:
 	#!/bin/sh
 
 	modprobe rcutorture
-	sleep 100
+	sleep 3600
 	rmmod rcutorture
 	dmesg | grep torture: