tools/memory-model/README: Mention herdtools7 7.56 in compatibility table

herdtools7 7.56 is going to be released in the week of 22 Jun 2020.
This commit therefore adds the exact version in the compatibility table.

Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>