adjust supported selectors doc section for block formatting

[bmizerany]

This commit changes the supported selectors doc section to render in a code block on pkgsite. Previously, the section was treated as a single bullet point and mushed together.

One downside of the Universal Selector is that is considered a bullet in GoDoc, which caused the formatting issue. This change also moves the mentioning of it the introduction of the block to avoid the issue.

[bmizerany]

Thank you very much for this wonderful package! I've been going to the docs for selector reference and kept finding it a little hard to read. Currently it looks like:

This change looks like:

The asterisk problem was a little hairy because gofmt keeps changing it to a - // Universal Selector to standardize it on - for a list over *. I could not find a way to convince godoc it was not in fact a list, so moving it out of the list was the only way I could come up with to fix the issue. I'm happy to change it to use some other way if anyone knows how to fix the issue.

[ericchiang]

Thanks for the fix!