Update leaflet tutorial

[arnesetzer]

:wave: I did some awesome work for the Pelias project and would love for everyone to have a look at it and provide feedback.

---

Here's the reason for this change :rocket:

Since the tutorial was outdated and the APIs didn't work anymore I decided to update the tutorial.

---

Here's what actually got changed :clap:

Just changed some urls and updated the commands according to: https://github.com/pelias/leaflet-plugin

---

Here's how others can test the changes :eyes:

Set up a own website with leaflet and try it out😅