Add command needed for :observer.start() within iex -S mix (#14904)

lib/elixir/pages/mix-and-otp/dynamic-supervisor.md

@@ -202,9 +202,10 @@ iex> :observer.start()
 
 > #### Missing dependencies {: .warning}
 >
-> When running `iex` inside a project with `iex -S mix`, `observer` won't be available as a dependency. To do so, you will need to call the following functions before:
+> When running `iex` inside a project with `iex -S mix`, `observer` won't be available as a dependency. To do so, you will need to call the following functions:
 >
 > ```elixir
+> iex> Mix.ensure_application!(:observer)
 > iex> :observer.start()
 > ```
 >