Update supervisor config in getting started guide (#338)

Author: amclain

guides/getting_started.md

@@ -76,15 +76,13 @@ defmodule MyApp do
   # ...
 
   def start(_type, _args) do
-    import Supervisor.Spec, warn: false
-
     # 1. Load the viewport configuration from config
     main_viewport_config = Application.get_env(:my_app, :viewport)
 
     # 2. Start the application with the viewport
     children = [
       # ...
-      supervisor(Scenic, [viewports: [main_viewport_config]]),
+      {Scenic, [main_viewport_config]},
     ]
 
     Supervisor.start_link(children, strategy: :one_for_one)