Construct a bid request type correct on readme

[JaosnHsieh]

Validation schema is correct but the sample code on README is wrong type.

Construct a bid request

  var bidRequestBuilder = openrtb.getBuilder({
    builderType: 'bidRequest'
  });
  var bidRequest = bidRequestBuilder
  .timestamp(moment.utc().format())
  .id('1234')
  .at(2)
  .imp([
    {
      "id":"1",
      "native":{
        "request": {
          "ver": 1,
          "layout": 6,
          "assets": [
            { "id": 0, "req": 1, "title": { "len": 25 } }, 
            { "id": 1, "req": 1, "img": { "type": 3, "wmin": 100, "hmin": 100 } },
            { "id": 3, "req": 0, "data": { "type": 2, "len": 90 } }
          ]
        }
      },
...

to

Construct a bid request

  var bidRequestBuilder = openrtb.getBuilder({
    builderType: 'bidRequest'
  });
  var bidRequest = bidRequestBuilder
  .timestamp(moment.utc().format())
  .id('1234')
  .at(2)
  .imp([
    {
      "id":"1",
      "native":{
        "request": JSON.stringify({
          "ver": 1,
          "layout": 6,
          "assets": [
            { "id": 0, "req": 1, "title": { "len": 25 } }, 
            { "id": 1, "req": 1, "img": { "type": 3, "wmin": 100, "hmin": 100 } },
            { "id": 3, "req": 0, "data": { "type": 2, "len": 90 } }
          ]
        })
      },