Improve instruction in regex match single character challenge

[Manish-Giri]

Pre-Submission Checklist

• Your pull request targets the staging branch of freeCodeCamp.
• Branch starts with either fix/, feature/, or translate/ (e.g. fix/signin-issue)
• You have only one commit (if not, squash them into one commit).
• All new and existing tests pass the command npm test. Use git commit --amend to amend any fixes.

Type of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)
• Add new translation (feature adding new translations)

Checklist:

• Tested changes locally.
• Closes currently open issue (replace XXXX with an issue no): Closes [beta] Improve Instruction for "Regular Expressions: Match Single Character with Multiple Possibilities" #13044

Description

Improved instruction and test case message in "Regular Expressions: Match Single Character with Multiple Possibilities" challenge.

Closes #13044

[Greenheart]

@Manish-Giri Thank you for fixing this! 👍