Fix typespec

Author: v0idpwn

lib/protobuf/presence.ex

@@ -15,7 +15,7 @@ defmodule Protobuf.Presence do
   always have implicit presence. In these cases, if the presence is ambiguous,
   returns `:maybe`.
   """
-  @spec field_presence(Protobuf.Message.t(), atom()) :: :present | :not_present | :maybe
+  @spec field_presence(message :: struct(), field :: atom()) :: :present | :not_present | :maybe
   def field_presence(%mod{} = message, field) do
     message_props = mod.__message_props__()
     transformed_message = transform_module(message, mod)