request.d.ts: fixing RequestAPI.Options.form type

In the definition file for the [request](https://github.com/mikeal/request) library, ```RequestAPI.Options.form```  is of type ```FormData```. ([Line 64](https://github.com/borisyankov/DefinitelyTyped/blob/b0b4af1223f4f9449012688fa0c18f20cc3267e7/request/request.d.ts#L64))

This prevents you from writing code like the example given in the request docs, because a plain object can't be used as a value for ```form```:
```javascript
request.post('http://service.com/upload', {form:{key:'value'}})
```

From reading the request docs and source code I don't think this type is right; it needs to be either a POJO or a string, so I've modified the type to ```any```