Add hint to not use LIMIT in "Simple aggregation"

[thomasboyt]

Noticed that this question is easily solvable without using the MAX function as intended:

select joindate as latest from cd.members order by joindate desc limit 1

This just adds an extra line telling answerers not to use LIMIT (similar to the hint on a previous question).

[andrei-polukhin]

Agree! The task is aggregation, so we should highlight not to use ORDER with LIMIT.