corrected a grammar mistake.

[TotallyCurious]

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)
• My pull request targets the master branch of freeCodeCamp.
• None of my changes are plagiarized from another source without proper attribution.
• My article does not contain shortened URLs or affiliate links.

[RandellDawson]

Hi @TotallyCurious

It seems that similar changes have already been accepted earlier for this article you're editing, sorry about that. 😓

If you feel you have more to add, please feel free to open up a new PR.

Thanks again! 😊

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