Fix broken code example in Protobuf.TransformModule docs (#383)

Author: v0idpwn

lib/protobuf/transform_module.ex

@@ -13,8 +13,6 @@ defmodule Protobuf.TransformModule do
       defmodule StringMessage do
         use Protobuf, syntax: :proto3
 
-        defstruct [:value]
-
         field :value, 1, type: :string
 
         def transform_module(), do: MyTransformModule