docs: Update step_02.ngdoc with ng-repeat challenge

I tried making a 5x5 table using two ng-repeaters. This showed me that it is the element with the ng-repeat attribute on it that gets repeated, and not only the contents of it. Next time I won't be adding the repeater to the table tag :)
Anyway, this is why I propose to add a little challenge to the readers regarding ng-repeat.