Update delete_at example to also use non-numeric values

lib/koans/07_lists.ex

@@ -24,7 +24,7 @@ defmodule Lists do
   end
 
   koan "Precision is also valued" do
-    assert List.delete_at([1, 2, 3], 1) == ___
+    assert List.delete_at([:a, :b, :c], 2) == ___
   end
 
   koan "Replication is also possible" do

test/koans/lists_koans_test.exs

@@ -8,7 +8,7 @@ defmodule ListsTests do
                [1, 2, :a, "b"],
                [1,2],
                [:a, :c],
-               [1,3],
+               [:a, :b],
                ["life", "life", "life"],
                [1, 2, 3, 4, 5],
                [1, 4, 2, 3],