Adds alternative solutions for an exercise

[pemocarlo]

This adds alternative solutions to the reading comprehension exercise Assignment
via advanced indexing of the chapter Advanced indexing from Module 3

[rsokl]

Thanks for this @CarlosPerezM ! I'll review this ASAP 😄

[rsokl]

This looks good! One minor suggestion, but this should be good to go.

[pemocarlo]

Thanks for the review!
And thanks for the great work!. I am really enjoying PLYMI, it has the right information to get up to speed with Python, quantity and quality wise. I also like the reading comprehension questions along the way.
I have some other ideas to add, so expect more PRs in the next days ;)

[rsokl]

Thanks for the kind feedback @CarlosPerezM ! And please do open those PRs - I am always looking to improve PLYMI, and am especially keen to add more reading comprehension problems and end-of-module problems. Also, let me know if you have any other feedback/critiques as you work through the site.