Fix List merge rule description

[cloneot]

Fix the following statement of List merge rules
from "Imagine we have some list [a, b] and say A inserts the sequence [d, e] after a whilst B inserts [f, g] after a"
to "Imagine we have some list [a, b] and say A inserts the sequence [d, e] after b whilst B inserts [f, g] after b".