add instructor callout for challenge

[avallecam]

After:

tutorials-early/episodes/describe-cases.Rmd

Lines 154 to 160 in 9128efa

	::::::::::::::::::::::::::::::::::::: challenge
	## Challenge 1: Can you do it?
	- **Task**: Aggregate `sim_data` linelist based on admission date and case outcome in __biweekly__
	intervals, and save the results in an object called `biweekly_incidence`.
	::::::::::::::::::::::::::::::::::::::::::::::::

Add text about

! `interval` must be one of:
    - an <integer> value;
    - 'day' or 'daily';
    - 'week(s)', 'weekly' or 'isoweek';
    - 'epiweek(s)';
    - 'month(s)', 'monthly', 'yearmonth';
    - 'quarter(s)', 'quarterly', 'yearquarter';
    - 'year(s)' or 'yearly'.