Update description of custom-redacted.css

This file is for the purpose of showing people how to change the background of their server,and don't know how to do CSS. The above, simply, is a tiling background. If your image is smaller than 1289x967 then the image will tile (1289x967 is the size of the background on the official Showdown server). To change the image, change the URL already there to the URL of the image you wish to set as the background. To change the background color (generally not seen), change the hex code #d37519 to the hex code of another color.