update gnoweb favicon

[thehowl]

cc/ @alexiscolin

https://github.com/gnolang/gno/blob/master/gno.land/pkg/gnoweb/static/img/favicon.ico

The current gnoweb icon is still the old gno logo rather than the black hole.

One option is to re-use the docs icon. However, maybe it makes sense for it to be something slightly different (ie. inverted colours) so the tabs can be distinguished?