Problem: Docs should mention the comma after `"builders": [...]`

While running through the getting-started examples, I ran into a strange
error:

	packer $ packer validate example.json
	Failed to parse template: Error in line 16, char 2: invalid character
	'"' after object key:value pair
	  "provisioners": [{
	packer $

The error was not immediately obvious, as this is my first time working
with Packer, and like many users this also happens to be my first time
working with JSON.

Solution:

I was missing a comma after the 'builders' configuration. I'm sure this
is a common problem, so let's specifically mention that in the document.
Merge branch 'master' of github.com:stefanlasiewski/packer