Fix documentation (DocTest) for Phoenix.PubSub (#119)

Use `Process.info(self(), :messages)` instead of `Process.info(self())[:messages]`.
The later one was returning an empty array for me every time.

lib/phoenix/pubsub.ex

@@ -49,12 +49,12 @@ defmodule Phoenix.PubSub do
 
       iex> PubSub.subscribe :my_pubsub, "user:123"
       :ok
-      iex> Process.info(self())[:messages]
-      []
+      iex> Process.info(self(), :messages)
+      {:messages, []}
       iex> PubSub.broadcast :my_pubsub, "user:123", {:user_update, %{id: 123, name: "Shane"}}
       :ok
-      iex> Process.info(self())[:messages]
-      {:user_update, %{id: 123, name: "Shane"}}
+      iex> Process.info(self(), :messages)
+      {:messages, [{:user_update, %{id: 123, name: "Shane"}}]}
 
   ## Implementing your own adapter