Update index.md

[nitik1998]

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[QuincyLarson]

Thank you for opening this pull request.

Book recommendations are beyond the scope of the freeCodeCamp Guide, so we've decided to go ahead and remove these book recommendation articles.

I encourage you to review this book in the ratings category of the freeCodeCamp forum. Your recommendation will reach way more people there.

Thanks again recommending this book, and for contributing to the freeCodeCamp Guide.