Fix date time links (#478)

* Fix date_time and date_time-package-source links.

* Fix link to source of date_time package.

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

Co-authored-by: mk-mxp <55182845+mk-mxp@users.noreply.github.com>

* Remove date-time package source link

Removed link to date-time package source from instructions (as it was only used in admonition).

* Change note about online editor to the new markup.

---------

Co-authored-by: mk-mxp <55182845+mk-mxp@users.noreply.github.com>

Author: oliolioli

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

@@ -3,7 +3,7 @@
 ## Working with Dates in Prolog
 
 Prolog's built-in support for dates is [fairly rudimentary][built-in-dates-support].
-While you can solve this exercise with just what's built into Prolog, a much nicer way to work with dates is via the [date_time package][date_time-package].
+While you can solve this exercise with just what's built into Prolog, a much nicer way to work with dates is via the [date_time package][date-time-package].
 
 ### Installing the date_time package
 
@@ -19,9 +19,9 @@ If that doesn't work, you can run:
 swipl -g 'pack_install(date_time)'
 ```
 
-```exercism/note
+~~~~exercism/note
 If you're using the online editor, you don't need to manually install anything.
-```
+~~~~
 
 ### Using the date_time package
 
@@ -31,10 +31,11 @@ Ad the following code to the top of your solution file to use the `date_time` pa
 :- use_module(library(date_time)).
 ```
 
-```exercism/note
-Tip: the [source of the date_time package][https://www.swi-prolog.org/pack/file_details/date_time/prolog/date_time.pl?show=src] is well documented and be a great help!
-```
+~~~~exercism/note
+Tip: the [source of the date_time package][date-time-package-source] is well documented and be a great help!
+
+[date-time-package-source]: https://www.swi-prolog.org/pack/file_details/date_time/prolog/date_time.pl?show=src
+~~~~
 
 [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
-[date_time-package-source]: https://www.swi-prolog.org/pack/file_details/date_time/prolog/date_time.pl?show=src
+[date-time-package]: https://www.swi-prolog.org/pack/list?p=date_time