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>