Update step_05.ngdoc with JSONLint validation hint

Added suggestion to use JSONLint to validate the JSON file and identify errors. It is simpler to address JSON errors independently of other errors at compile-time.

Motivation was based on personal experience following an applied version of Step 5 of the AngularJS Tutorial using Ionic and incorrectly placing a semi-colon at the end of the JSON file, such that both of the following approaches resulted in errors and data was not displayed in the browser: 
```
$scope.sentences = data; 
```
```
$scope.sentences = angular.fromJson(data); 
```
Solution to the problem (i.e. using JSONLint that identified the incorrectly used semi-colon in the JSON file) was found thanks to a response to my StackOverflow post [Here](http://stackoverflow.com/questions/25829315/deserialise-json-string-to-object-with-angularjs-using-generate-ionic)