Potential problem with documentation

[SLsthompson]

In the documentation (https://hexdocs.pm/phoenix_pubsub/Phoenix.Tracker.html) under the heading "Implementing a Tracker" where it does:

defmodule MyTracker do
  @behavior Phoenix.Tracker
  ...

It then goes on to define a start_link that calls GenServer.start_link

Should that sample code not include use GenServer as well? Without it, the module does not define child_spec and, like #94, problems with the deprecation of Supervisor.Spec ensue.

(if the change is appropriate, I would be happy to put together a pull request to make it)

[chrismccord]

Please send a PR to include child_spec in the example under that heading. Thanks!

[josevalim]

Fixed, thanks!