Update index.jsx,, changed expression from "." to "x" in multiplication

[Harshan-Nayak]

while multiplication , i changed symbol from "." to "x".

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• [x] I have read freeCodeCamp's contribution guidelines.
• [x] My pull request has a descriptive title (not a vague title like Update index.md)

Closes #XXXXX

[GuillermoFloresV]

Closing as there is no issue associated with this MR. If you see improvements that you would like to make to this in the future, please create a well documented issue and justify the changes being made. Thanks!