Prefer function output over input

When learning about the behavior of a function, in general the learning
experience is better when provided with the input and asked for the
output.

lib/koans/02_strings.ex

@@ -32,10 +32,10 @@ defmodule Strings do
   end
 
   koan "Repetition is the mother of learning" do
-    assert "StringStringString" == String.duplicate(___, 3)
+    assert String.duplicate("String", 3) == ___
   end
 
   koan "Strings can be louder when necessary" do
-    assert "LISTEN" == String.upcase(___)
+    assert String.upcase("listen") == ___
   end
 end