Change assignment link in DOM_manipulation_and_events.md

[scuddi]

Because

Old link linked to the second active learning at MDN, new link links to the first active learning which gives the foundation for the second exercise.

I had problems starting with the second exercise, doing the first before helped me a lot and made the second a lot easier.

If this was intended and the link stays the same I would recommend to write at least a hint to the first active learning or state specifically to start with the shopping list.

This PR

• Changed the link/anchor under ### Assignment which links to MDNs Active Learning section on DOM manipulation.

• Old link/anchor: https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Manipulating_documents#active_learning_a_dynamic_shopping_list

• New link/anchor: https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Manipulating_documents#active_learning_basic_dom_manipulation

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