feat(koan): add a koan about rounding pitfalls

lib/koans/03_numbers.ex

@@ -109,6 +109,10 @@ defmodule Numbers do
     assert Float.round(-5.5674, 3) === ___
   end
 
+  koan "But beware of floating dragons" do
+    assert Float.round(2.675, 2) == ___
+  end
+
   koan "I want the first and last in the range" do
     first..last = Range.new(1, 10)
 

test/koans/numbers_koans_test.exs

@@ -28,6 +28,7 @@ defmodule NumbersTests do
       99.0,
       12.34,
       {:multiple, [6.0, 5.0, 8.9, -5.567]},
+      2.67,
       {:multiple, [1, 10]},
       {:multiple, [true, true, false]},
       {:multiple, [true, false]}