Fix broken link in async.md in CC.AI studio

[mrutkows]

101-workshop-nodejs/ex1-creating-and-invoking-actions/async.md

has a callout box with a link to a bonus exercise:

{% hint style="info" %}
Bonus exercise: Try out this exercise [Calling an external API from an Async action](../bonus-exercises/ex1-invoking-an-async-api.md) to test your knowledge!
{% endhint %}

which is not in CC.AI studio

[mrutkows]

The page is valid: 101-workshop-nodejs/bonus-exercises/ex1-invoking-an-async-api.md

but, there is no place to locate "bonus" content logically in CC.AI