rcutorture: Fix incorrect description of default for nreaders parameter

The comment for the nreaders parameter of rcutorture gives the default as
4*ncpus, but the value actually defaults to 2*ncpus; fix the comment.

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Acked-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>