[Typos?] Step 15

In [Step 15](https://github.com/mozdevs/html5-games-workshop/blob/master/src/content/platformer/step15_en.md)

> [...] then switch to #1 when #0 is completed. **Once the last level (in this case, #1) [*] we should present** some kind of "game completed" screen [...]

Seems like a word is missing somewhere there?

> [...] What we would **to** instead is to restart [...]

**like to do**?