Fix formatting for z-index definition

[valentin-radulescu-hs]

It reads:

Defines the order of the elements on the z-axis*8. It only works on positioned elements (anything apart from static).

It should probably read:

Defines the order of the elements on the z-axis. It only works on positioned elements (anything apart from static).

[MirShaon]

👍

[fredericksilva]

Fixed #1

[jgthms]

Thanks!