Consistent usage of double quotes in documentation.

Since all bundled configs use double quotes, I think it's a good idea to use them in the documentation as well.

Plus `Acceptable values` for [`leading-zero`][1] were showing up as code because the list is indented 4 spaces.

[1]: https://github.com/csscomb/csscomb.js/blob/master/doc/options.md#leading-zero