Change definition of anonymous greet

That prevents the confusion of mixing up the anonymous and named
implementations fixed by 55f592.

lib/koans/13_functions.ex

@@ -68,7 +68,7 @@ defmodule Functions do
   end
 
   koan "Prefix a string with & to build a simple anonymous greet function" do
-    greet = &"Hello, #{&1}!"
+    greet = &"Hi, #{&1}!"
     assert greet.("Foo") == ___
   end
 

test/koans/functions_koans_test.exs

@@ -13,7 +13,7 @@ defmodule FunctionsTests do
       {:multiple, ["The number was zero", "The number was 5"]},
       6,
       6,
-      "Hello, Foo!",
+      "Hi, Foo!",
       ["foo", "foo", "foo"],
       100,
       1000,