Move sigils to later in the lessons

While doing this, I also discovered that
there is also a reference to lists in numbers, but
that might a bit easier for someone to grasp, given
the hint that the koan gives, and the output they
see when they run it.

lib/koans/02_strings.ex

@@ -15,7 +15,7 @@ defmodule Strings do
     assert "hello world" == ___ <> "world"
   end
 
-  koan "Or pulled apart when needed" do
+  koan "Or pulled apart into a list when needed" do
     assert ["hello", "world"] == String.split(___, " ")
   end