Fix SPDY example in README

Using the original example didn't work for me, but the this amended version does. I'm not sure when this stopped working (or I'm doing something completely daft).

Here is the example script I used.

```js
var http, proxiedHttp, proxywrap, server, spdyOpts;

http = require('spdy');

proxywrap = require('proxywrap');

proxiedHttp = proxywrap.proxy(http.server, {
  strict: false
});

spdyOpts = {
  plain: true,
  ssl: false
};

var app = function(req, res){};
server = proxiedHttp.create(spdyOpts, app);
server.listen(8888);
```

and here is the error I got with the example from the readme

```
tls.js:1122
    throw new Error('Missing PFX or certificate + private key.');
          ^
Error: Missing PFX or certificate + private key.
  at ProxiedServer.Server (tls.js:1122:11)
  at ProxiedServer.Server (https.js:35:14)
```