Update computed-properties-and-aggregate-data.md

A code example of an array being filtered when the array only has one example is not terribly interesting (and probably doesn't get the point across as much). My suggestion is to make the example a little more interesting, while keeping it relatively simple still.
Update computed-properties-and-aggregate-data.md

Modified to go along with my other change
Merge pull request #1489 from kalmanh/patch-2

Update computed-properties-and-aggregate-data.md