Fix server icon saving example

[ItsDrike]

The original example we had in FAQ doesn't work for multiple reasons:

1. base64.decodebytes function expects bytes as the input arg, however status.icon is a string, we should be using base64.b64decode instead.
2. The base64 data is prefixed with "data:image/png;base64,", which isn't valid base64 data, and should not be a part of the saved PNG, it's just a a prefix informing us what kind of image this is, and what is the formatting (and a fairly standard one, even browsers can interpret it, but it certainly shouldn't be interpreted as part of the base64 encoded image data).
3. There was a typo in the variable holding the file pointer from context manager (server_icon_file) and the variable used in body (server_favicon_file).

The snippet originally also didn't contain the code that originally obtained this image, I've included that to make it very clear where the status variable came from.