rcu: Remove obsolete Kconfig option from comment

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

include/linux/rcupdate.h

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 275aa3f..7e12dba 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -749,7 +749,7 @@
  * preemptible RCU implementations (TREE_PREEMPT_RCU and TINY_PREEMPT_RCU)
  * in CONFIG_PREEMPT kernel builds, RCU read-side critical sections may
  * be preempted, but explicit blocking is illegal.  Finally, in preemptible
- * RCU implementations in real-time (CONFIG_PREEMPT_RT) kernel builds,
+ * RCU implementations in real-time (with -rt patchset) kernel builds,
  * RCU read-side critical sections may be preempted and they may also
  * block, but only when acquiring spinlocks that are subject to priority
  * inheritance.