Add source_url

Co-authored-by: Erik Schierboom <erik_schierboom@hotmail.com>

Author: atk

exercises/jigsaw-puzzle/metadata.toml

@@ -1,3 +1,4 @@
 title = "Jigsaw Puzzle"
 blurb = "Fill in missing jigsaw puzzle details from partial data"
-source = "atk just started another 1000-pieces jigsaw puzzle when this idea hit him"
+source = "atk just started another 1000-pieces jigsaw puzzle when this idea hit him"
+source_url = "https://github.com/exercism/problem-specifications/pull/2554"