rcutorture: Rename rcutorture_runnable parameter

This commit changes rcutorture_runnable to torture_runnable, which is
consistent with the names of the other parameters and is a bit shorter
as well.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/kernel-parameters.txt

@@ -2938,7 +2938,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Set time (s) between CPU-hotplug operations, or
 			zero to disable CPU-hotplug testing.
 
-	rcutorture.rcutorture_runnable= [BOOT]
+	rcutorture.torture_runnable= [BOOT]
 			Start rcutorture running at boot time.
 
 	rcutorture.shuffle_interval= [KNL]