Fix link to source of date_time package.

Author: oliolioli

exercises/practice/gigasecond/.docs/instructions.append.md

@@ -31,9 +31,9 @@ Ad the following code to the top of your solution file to use the `date_time` pa
 :- use_module(library(date_time)).
 ```
 
-```exercism/note
+### Exercism note
 Tip: the [source of the date_time package][date-time-package-source] is well documented and be a great help!
-```
+
 
 [built-in-dates-support]: https://www.swi-prolog.org/pldoc/man?section=timedate
 [date-time-package]: https://www.swi-prolog.org/pack/list?p=date_time