Update test case in Increment a Number with JavaScript

[AngShiYa]

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 Increment a Number with JavaScript has a misleading test case #12593

Description

Update test case from "myVar = myVar should be changed" to "myVar = myVar + 1; should be changed"

[raisedadead]

✨ ✨ LGTM. ✨ ✨

Thanks @AngShiYa !

Congratulations on your first PR to freeCodeCamp!