[json/en] JSON should not use single quote delimited strings

Although there may be parsers that allow the use of single quotes, it is definitely non-standard.

[JSON.org](http://json.org/) reads:

> A string is a sequence of zero or more Unicode characters, wrapped in double quotes, using backslash escapes.

Also, [JSONLint](http://jsonlint.com/) will complain about anything delimited by single quotes.

```javascript
JSON.parse("{'key' : 'value'}");
```
will throw `SyntaxError: Unexpected token '`