docs: remove reference to JSON3

The `json3` dependency, previously used for compatibility with IE6/7,
was removed in [1] and included in `socket.io-parser@3.0.0` and
`socket.io-client@2.0.0` ([2]).

The slim build now only removes the debug dependency (~3KB min+gzip).

[1] socketio/socket.io-parser@e39f5a8#diff-b9cfc7f2cdf78a7f4b91a753d10865a2
[2] ff4cb3e#diff-b9cfc7f2cdf78a7f4b91a753d10865a2R28

README.md

@@ -33,7 +33,7 @@ import io from 'socket.io-client';
 const socket = io();
 ```
 
-A slim build (without `JSON3`, a JSON polyfill for IE6/IE7, and `debug`) is also available: `socket.io.slim.js`.
+A slim build (without [debug](https://github.com/visionmedia/debug)) is also available: `socket.io.slim.js`.
 
 Socket.IO is compatible with [browserify](http://browserify.org/) and [webpack](https://webpack.js.org/) (see example [there](https://github.com/socketio/socket.io/tree/2.0.3/examples/webpack-build)).