Tweak JSON example of unmarshalling a float

The current example shows `json.Unmarshal` on an array of bytes, including
`num:6.0`. In the output however this comes out as `6`. Even in the casting to
`float64`, it comes out as `6`. This can be confusion because it looks like a
simple `int`. By adding an arbitray `.13` you see it's a
float.

examples/json/json.hash

@@ -1,2 +1,2 @@
-12f985bb444e2bd6d72ea52a24ebc5f4f82c5b9b
-4L2wMVv7tW
+9a3d67892b3930bceb1167b59d4d4f5527dadefc
+scgxBwACYx