Phoenix walkthrough doc update (bitwalker#713)

* Updated default Phoenix config to reflect latest version
* Added steps to generate a secret key base for Phoenix

docs/guides/phoenix_walkthrough.md

@@ -33,7 +33,6 @@ First let's modify the Phoenix `config/prod.exs` file. Change this section of te
 
 ```elixir
 config :phoenix_distillery, PhoenixDistilleryWeb.Endpoint,
-  http: [:inet6, port: System.get_env("PORT") || 4000],
   url: [host: "example.com", port: 80],
   cache_static_manifest: "priv/static/cache_manifest.json"
 ```
@@ -42,14 +41,35 @@ to the following:
 
 ```elixir
 config :phoenix_distillery, PhoenixDistilleryWeb.Endpoint,
-  http: [:inet6, port: System.get_env("PORT") || 4000],
-  url: [host: "localhost", port: System.get_env("PORT")], # This is critical for ensuring web-sockets properly authorize.
+  http: [:inet6, port: {:system, "PORT"}],
+  url: [host: "localhost", port: {:system, "PORT"}], # This is critical for ensuring web-sockets properly authorize.
   cache_static_manifest: "priv/static/cache_manifest.json",
   server: true,
   root: ".",
   version: Application.spec(:phoenix_distillery, :vsn)
 ```
 
+We also need to change the secret key base in `config/prod.secret.exs` . Change this section of text:
+
+```elixir
+secret_key_base =
+  System.get_env("SECRET_KEY_BASE") ||
+    raise """
+    environment variable SECRET_KEY_BASE is missing.
+    You can generate one by calling: mix phx.gen.secret
+    """
+```
+
+to the following:
+
+```elixir
+secret_key_base =
+  "5U8dBbveeM1DMJtFZq6Ybaum394cVHDHHj/YnKo8r8461WS9eFDWT2YpLzuODsan"
+```
+
+**NOTE** The secret key base should be generated using `mix phx.gen.secret`. It
+should also not be committed to your VCS in plain text.
+
 Let's discuss these options.
 
 - `server` configures the endpoint to boot the [Cowboy](https://github.com/ninenines/cowboy) application http endpoint on start.