fix(challenges): fix description in css variable fallback challenge

[joshalling]

Description

I fixed the description in this challenge to point out that the variable fallback is not used to work with browsers that don't support variables.

I also updated the instruction to call out exactly what is going on rather than explaining it as a note at the bottom.

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)

PARTIALLY RESOLVES: freeCodeCamp/freeCodeCamp#17546

[scissorsneedfoodtoo]

@joshalling, thank you for improving this challenge! It's much more clear what a fallback is and what the user is meant to do in this challenge.