Improve ch03-05-control-flow collection looping wording

[RafaelKr]

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This makes it clearer, that you COULD use a while loop to loop a collection, but a for loop is the preferred way

When reading the section "Looping Through a Collection with for" for the first time this morning I was confused it starts with "You can choose to use the while construct to loop over the elements of a collection..." For me it cleared up when reading further.

But also #3757 was created by @Atabic today who also got confused by this. So I think the wording may be improved by using "could" instead of "can".