Update leaflet tutorial

[arnesetzer]

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

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Here's the reason for this change 🚀

Since the tutorial was outdated and the APIs didn't work anymore I decided to update the tutorial.

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Here's what actually got changed 👏

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

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Here's how others can test the changes 👀

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