doc: fix default maxHeadersCount

[v0.8](https://github.com/joyent/node/blob/v0.8/lib/http.js#L1643), [v0.10](https://github.com/joyent/node/blob/v0.10/lib/http.js#L1750), and [master](https://github.com/joyent/node/blob/master/lib/_http_server.js#L341) use a default of 2000 for maxHeadersCount.