Clarify reference to all_equal function

https://poeditor.com/projects/po_edit?id=490053&term=344292569&id_language=50#showComments344292569

> A French user report this : "Il serait préférable de ne pas citer la fonction tous_egaux à ce moment là de l’exercice car elle n’est pas exploitable. La seconde solution donnée en fin d’exercice ne marche pas, car la fonction tous_egaux n’est pas définie." Could you check where could be the issue ; it seems all about `all_equal` is translated...

Hmmm, this is a fair point. `all_equal` was written two pages earlier. Users may skip to this page directly, or they may have just forgotten about it. Besides that, `assert_equal` gets special treatment and is always defined, so they may expect the same for `all_equal` and be surprised by getting an error. I think when this content was written the first time, this might have been less of an issue.

I think encouraging reuse of functions and connecting to previous learning is good, but maybe the text should make it clearer what `all_equal` is referring to (e.g. with a link to the page) and make it clear that it's not *required* to solve the current exercise.