Update introduction-a-heurist.md

[anisa-hawes]

I'm updating broken links in the FR lesson introduction-a-heurist (at lines 81 and 868).

Closes #3592

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[anisa-hawes]

Bonjour @marie-flesch,

A couple of quick link fixes in introduction-a-heurist.

These fixes reflect that the website https://heuristnetwork.org/ has been moved - so the location of their Installation guide and Tutorials has been updated.

[marie-flesch]

All good :)

[marie-flesch]

Thank you for this @anisa-hawes!