Modify Even Odd in Pyret

[rzuckerm]

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I Am Modifying an Existing Code Snippet or Existing Tests

• I fixed Modify Even Odd in Pyret #3686
• I have read about modifying existing code snippets or tests in the contributing documentation
• I have explained why this modification is necessary either in the issue linked above or in the notes below
• I named the pull request using Modify {PROJECT} {tests} in {LANGUAGE} format

Other Notes

I realized that I forgot to check if the number is an integer.

[jrg94]

Easy enough!