rcutorture: Correct name of use_softirq module parameter

The BUSTED-BOOST and TREE03 scenarios specify a mythical tree.use_softirq
module parameter, which means a failure to get full test coverage.  This
commit therefore corrects the name to rcutree.use_softirq.

Fixes: e2b949d5 ("rcutorture: Make TREE03 use real-time tree.use_softirq setting")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>