rcu: Fix typo and add additional debug

This commit fixes a typo and adds some additional debugging to the
message emitted when a task blocking the current grace period is listed
as blocking it when either that grace period ends or the next grace
period begins.  This commit also reformats the console message for
readability.

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