Warn about non-breaking space in AboutDates

[DavidWhitlock]

In newer versions of the JRE, the date formatting APIs return strings containing non-breaking spaces. This is surprising to students who do the AboutDates koans because the copy and paste from the command line doesn't necessarily work.

Let's update AboutDates to mention that there is a non-breaking space before the AM/PM.

Let's also update the koan to use a consistent time zone for the date object and not rely on ZoneId.systemDefault() because it can vary between systems.