Update README

Hi *Daniel Friesen*, :smile:

I correct the **word spelling** for you. :pencil:

Before: `require_once("$IP/skins/monaco/ContentRightSiebar.php");`
Now: `require_once("$IP/skins/monaco/ContentRightSidebar.php");`


Yours,
[**Suriyaa Kudo**](https://github.com/SuriyaaKudoIsc)