Add Fibonacci to Fortran

[Butenkite]

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I Am Adding a New Code Snippet in an Existing Language

• I fixed Add Fibonacci in Fortran #4472
• I did not include any extra folders/libraries
• I named the pull request using Add {PROJECT} in {LANGUAGE} format

[github-actions]

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[Kaamkiya]

Nice work! I just have one change I'd like to request.

Your program checks for ASCII letters, but what about others invalid characters, like "$", "@", etc?

Please check for those characters as well.

Once you've fixed that, I'll merge this PR.

Thank you!

[rzuckerm]

This is consistent with how out Fortran samples validate the input.