Adds plug :action to controller

I'm beginning to work through the guides, couldn't get the page to render and finally chased it down to this line.
correct and explicitly specify language
Merge pull request #35 from edubkendo/patch-1

Adds plug :action to controller
Merge pull request #36 from Joe-noh/jh-correct-language-name

correct and explicitly specify language
adds a blank line to match what Phoenix generates
adds a description of plug :action and what it does; adds missing plug :action line to HelloPhoenix.HelloController definition
Merge pull request #39 from lancehalvorsen/add-plug-action-explanation-to-controller-guide

Add plug action explanation to controller guide
ultimately, plug calls to the Plug.Builder module, but the closer definition is in phoenix.Controller.Pipeline