Clarify List.delete example

Using numbers in the example tends to confuse the learner about what the
purpose of the second argument to delete is. They might think it's an
index value. This is further exacerbated by the example's value at index
2 being 2. To clear this up, change the example to use non-numeric
values.

test/koans/lists_koans_test.exs

@@ -7,7 +7,7 @@ defmodule ListsTests do
                3,
                [1, 2, :a, "b"],
                [1,2],
-               [1,2,3],
+               [:a, :c],
                [1,3],
                ["life", "life", "life"],
                [1, 2, 3, 4, 5],