tools/memory-model: Use cumul-fence instead of fence in ->prop example

tools/memory-model: Use cumul-fence instead of fence in ->prop example

To reduce ambiguity in the more exotic ->prop ordering example, this
commit uses the term cumul-fence instead of the term fence for the two
fences, so that the implict ->rfe on loads/stores to Y are covered by
the description.

Link: https://lore.kernel.org/lkml/20190729121745.GA140682@google.com
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>