Add missing behaviour to custom-producers guide (#357) (#358)

Closes #357

Author: Krapaince

guides/examples/custom-producers.md

@@ -38,6 +38,8 @@ generated by the producer into proper Broadway messages:
     defmodule MyBroadway do
       use Broadway
 
+      @behaviour Broadway.Acknowledger
+
       alias Broadway.Message
 
       def start_link(_opts) do
@@ -65,8 +67,10 @@ generated by the producer into proper Broadway messages:
         }
       end
 
+      @impl Broadway.Acknowledger
       def ack(:ack_id, successful, failed) do
         # Write ack code here
+        :ok
       end
     end