exercises/concept/windowing-system/.docs/instructions.md typo

[kroitor]

Related to #1600

[github-actions]

Dear kroitor

Thank you for contributing to the JavaScript track on Exercism! 💙
You will see some automated feedback below 🤖. It would be great if you can make sure your PR covers those points. This will save your reviewer some time and your change can be merged quicker.

• ✍️ If your PR is not related to an existing issue (and is not self-explaining like a typo fix), please make sure the description explains why the change you made is necessary.

• 🔤 If your PR fixes an easy to identify typo, if would be great if you could check for that typo in the whole repo. For example, if you found Unicdoe, use "replace all" in your editor (or command line magic) to fix it consistently.

Dear Reviewer/Maintainer

• 📏 Make sure you set the appropriate x:size label for the PR. (This also works after merging, in case you forgot about it.)

• 🔍 Don't be too nit-picky. If the PR is a clear improvement compared to the status quo, it should be approved as clear signal this is good to be merged even if the minor comments you might have are not addressed by the contributor. Further improvement ideas can be captured in issues (if important enough) and implemented via additional PRs.

• 🤔 After reviewing the diff in the "Files changed" section, take a moment to think about whether there are changes missing from the diff. Does something need to be adjusted in other places so the code or content stays consistent?

_{Automated comment created by PR Commenter 🤖.}

[junedev]

Good catch! Thanks for fixing this.