Be more positive in string koans

lib/koans/03_strings.ex

@@ -14,7 +14,7 @@ defmodule Strings do
   end
 
   koan "Be careful, a message may be altered" do
-    assert :__ == String.replace("An incredible day", "incredible", "awful")
+    assert :__ == String.replace("An awful day", "awful", "incredible")
   end
 
   koan "But strings never lie about themselves" do

test/koans_harness_test.exs

@@ -34,7 +34,7 @@ defmodule KoansHarnessTest do
       "hello",
       "hello ",
       ["hello", "world"],
-      "An awful day",
+      "An incredible day",
       "incredible",
       "banana",
       "banana",