README update

Was still mentioning 2000 char limit on long URLs. See  https://twitter.com/hidgw/statuses/323347891362070528
Remove notice that has probably been here for decades.

<b>Warning</b>:  Illegal string offset '31.8888888888' in <b>D:\yourls\includes\functions-formatting.php</b> on line <b>21</b>