Docs: include use of fetch() in Stream-based example

lib/constructor.js

@@ -40,14 +40,16 @@ const debuglog = util.debuglog('sharp');
  *   });
  *
  * @example
- * // Read image data from readableStream,
+ * // Read image data from remote URL,
  * // resize to 300 pixels wide,
  * // emit an 'info' event with calculated dimensions
  * // and finally write image data to writableStream
- * var transformer = sharp()
+ * const { body } = fetch('https://...');
+ * const readableStream = Readable.fromWeb(body);
+ * const transformer = sharp()
  *   .resize(300)
- *   .on('info', function(info) {
- *     console.log('Image height is ' + info.height);
+ *   .on('info', ({ height }) => {
+ *     console.log(`Image height is ${height}`);
  *   });
  * readableStream.pipe(transformer).pipe(writableStream);
  *