Fixed Typo in Step 43 of Building a Cipher

[Jaiharishan]

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• I have read and followed the contribution guidelines.
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• My pull request targets the main branch of freeCodeCamp.
• I have tested these changes either locally on my machine, or GitPod.

Closes #XXXXX

[ilenia-magoni]

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[camperbot]

This PR seems to make similar changes as an existing PR. As such, we are going to close this as a duplicate.

If you feel you have additional changes to expand upon this PR, please feel free to push your commits and request this PR be reopened.

Thanks again! 😊

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