Fix data type of TransformedData.fields

[aeharvlee]

The real type of Transfromed.fields is Object, not string.
Currently we cannot use official example.

  const { ElasticsearchTransformer } = require('winston-elasticsearch');
  const esTransportOpts = {
  transformer: (logData) => {
   const transformed = ElasticsearchTransformer(logData);
   transformed.fields.customField = 'customValue'
   return transformed;
 }};
const esTransport = new ElasticsearchTransport(esTransportOpts);

Cannot use transformed.fields.customField = 'customValue' because transformed.fields is a string type, so TypeScript compiler throw error about this when do compile.

As you can see, the real data type of TransformedData.fields is Object, not string.