fix(challenges): rephrased wording in applied visual design

[TomerPacific]

Description

In order to clarify the meaning in the second paragraph of the challenge, the text has been changed to say:

One key difference between the fixed and absolute positions is that an element with a fixed position won't move when the user scrolls.

Pre-Submission Checklist

• [x] Your pull request targets the dev branch.
• [x] Branch starts with either fix/, feature/, or translate/ (e.g. fix/challenge-tests)
• [x] All new and existing tests pass the command npm test.
• [x] Use npm run commit to generate a conventional commit message. Learn more here: https://conventionalcommits.org/#why-use-conventional-commits
• [x] The changes were done locally on your machine and NOT GitHub web interface. If they were done on the web interface you have ensured that you are creating conventional commit messages.

Checklist:

• [x] Tested changes locally.
• [x] Addressed currently open issue (replace XXXXX with an issue no in next line)

Closes #18043

[joshalling]

@TomerPacific Thank you for helping to resolving freeCodeCamp/freeCodeCamp#18043!

And @Nirajn2311 thanks for your help in QAing.