[Typos?] Step 16

In [Step 16](https://github.com/mozdevs/html5-games-workshop/blob/master/src/content/platformer/step16_en.md)

> [...] "enter the door" animation for when a level **it's** finished.

> [...] You can keep **an** sprite in the world [...]

> [...] Phaser.Camera has methods for **fadign** in and out.