listRCU.rst document gives an example with 'ipc_lock()', but the
function has dropped off by commit 82061c57 ("ipc: drop
ipc_lock()").  Because the main logic of 'ipc_lock()' has melded in
'shm_lock()' by the commit, this commit updates the document to use
'shm_lock()' instead.
Reviewed-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>