rcu: Disambiguate CONFIG_RCU_NOCB_CPUs

This commit fixes a grammar issue in the rcu_nohz_full_cpu() comment
header, so that it is clear that the plural is CPUs not Kconfig options.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>