Fix syntax in post_login callback registration example

[MaxPleaner]

The earlier syntax didn't work. It said that a function was expected and a tuple was given. Doing the fix shown here made it work.

My deps for reference

[{:phoenix, "~> 1.2.1"},
 {:phoenix_pubsub, "~> 1.0"},
 {:phoenix_ecto, "~> 3.0"},
 {:postgrex, ">= 0.0.0"},
 {:phoenix_html, "~> 2.6"},
 {:phoenix_live_reload, "~> 1.0", only: :dev},
 {:gettext, "~> 0.11"},
 {:addict, "~> 0.3"},
 {:guardian, "~> 0.14"},
 {:mailgun, github: "chrismccord/mailgun", branch: "master", override: true},

 {:cowboy, "~> 1.0"}]

[trenpixster]

Thanks for looking into this 👍