Use dppx instead of non-existing ppx resolution unit

Commit 4f4d77d36e9f42cbfe406d87c0ef2b809f394c9e introduced a bug by using an unit called "ppx" which does not even exist. It should be written "dppx", like stated in bug #1964. More information can also be found [here](https://developer.mozilla.org/de/docs/Web/CSS/resolution) and [here](http://www.w3.org/TR/css3-values/#resolution).