doc/RCU/rcu: Use https instead of http if possible

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/rcu.rst

@@ -79,7 +79,7 @@ Frequently Asked Questions
   Of these, one was allowed to lapse by the assignee, and the
   others have been contributed to the Linux kernel under GPL.
   There are now also LGPL implementations of user-level RCU
-  available (http://liburcu.org/).
+  available (https://liburcu.org/).
 
 - I hear that RCU needs work in order to support realtime kernels?