doc: Update RCU's requirements page about the PREEMPT_RT wiki

The PREEMPT_RT wiki moved from kernel.org to the Linux Foundation wiki.
The kernel.org wiki is read only.

This commit therefore updates the URL of the active PREEMPT_RT wiki.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/Design/Requirements/Requirements.rst

@@ -2317,7 +2317,7 @@ decides to throw at it.
 
 The Linux kernel is used for real-time workloads, especially in
 conjunction with the `-rt
-patchset <https://rt.wiki.kernel.org/index.php/Main_Page>`__. The
+patchset <https://wiki.linuxfoundation.org/realtime/>`__. The
 real-time-latency response requirements are such that the traditional
 approach of disabling preemption across RCU read-side critical sections
 is inappropriate. Kernels built with ``CONFIG_PREEMPT=y`` therefore use