Resources documentation is missing placeholder

[voger]

In the page about adding resources, the line about importing assets from the theme is missing it's placeholder. The placeholder is there in the markdown file

https://github.com/qooxdoo/documentation/blob/master/desktop/gui/resources.md in line 95 but it doesn't show in the web page

https://qooxdoo.org/documentation/#/desktop/gui/resources

[hkollmann]

Could you add a PR to fix it? Thanks!