Update exercises/jigsaw-puzzle/introduction.md

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

Author: atk

exercises/jigsaw-puzzle/introduction.md

@@ -1,5 +1,5 @@
 # Introduction
 
-Your best friend has started collecting jigsaw puzzles and wants to build a detailed catalogue for the collection — recording information like the number of pieces, rows, columns, border pieces, inside pieces, aspect ratio, and the puzzle's format.
+Your best friend has started collecting jigsaw puzzles and wants to build a detailed catalogue for their collection — recording information like the number of pieces, rows, columns, border pieces, inside pieces, aspect ratio, and the puzzle's format.
 After solving one of the puzzles to figure out these details, you're now helping fill in the gaps for others in the collection, even when some data is missing.