MAINTAINERS: Update RCU git tree

RCU now has a shiny new shared git tree, so this commit updates
MAINTAINERS to point to it instead of my personal -rcu tree.

Reported-by: Song Liu <song@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>