fix incorrect trait bound demonstration

[woofyzhao]

Motivation

• The trait bound issue should be demonstrated using generic function rather than method in this demo.
• Method area is not defined on _triangle . Uncomment the line and compile it gives a method not defined error rather than anything related to generic type bound issue.
• Besides, the compiler generates a fn area unused warning which is not consistent with the rest of the book.

[rustbot]

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[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @marioidival (or someone else) soon.