Update book_controller.go comment message

[p3nj]

What this PR is changing or adding?

Comment message of CreateBook() at line 193 is mis-typed with Delete book by given ID.

Which issues are fixed by this PR?

Yes, modified a comment message at book_controller.go:193

Pre-launch Checklist

• [#] I have read and fully accepted project's code of conduct.
• [ ] I listed at least one issue that this PR fixes in the description above.
• [#] I updated/added relevant documentation and/or comments to the code.
• [#] All existing and new tests are passing successfully.

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