doc: Fix broken RCU-requirements link to LKML archive

Two of the Requirements.html LKML links are broken. This patch changes
them to use the archive from lore.kernel.org, which works fine.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>