Use assets.exercism.org subdomain (#658)

As part of moving various parts of our image hosting behind a CDN, this commit updates the involved urls we automatically found in this repository.

exercises/practice/killer-sudoku-helper/.docs/instructions.md

@@ -56,8 +56,8 @@ The screenshots above have been generated using [F-Puzzles.com](https://www.f-pu
 
 [sudoku-rules]: https://masteringsudoku.com/sudoku-rules-beginners/
 [killer-guide]: https://masteringsudoku.com/killer-sudoku/
-[one-solution-img]: https://exercism-v3-icons.s3.eu-west-2.amazonaws.com/images/exercises/killer-sudoku-helper/example1.png
-[four-solutions-img]: https://exercism-v3-icons.s3.eu-west-2.amazonaws.com/images/exercises/killer-sudoku-helper/example2.png
-[not-possible-img]: https://exercism-v3-icons.s3.eu-west-2.amazonaws.com/images/exercises/killer-sudoku-helper/example3.png
+[one-solution-img]: https://assets.exercism.org/images/exercises/killer-sudoku-helper/example1.png
+[four-solutions-img]: https://assets.exercism.org/images/exercises/killer-sudoku-helper/example2.png
+[not-possible-img]: https://assets.exercism.org/images/exercises/killer-sudoku-helper/example3.png
 [clover-puzzle]: https://app.crackingthecryptic.com/sudoku/HqTBn3Pr6R
 [goodliffe-video]: https://youtu.be/c_NjEbFEeW0?t=1180