Chapter 8.3 Improvement: Mentioning the tuple type

[intervall-daniel]

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• [x] I have checked the latest main branch to see if this has already been fixed
• [x] I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem: https://doc.rust-lang.org/book/ch08-03-hash-maps.html#accessing-values-in-a-hash-map

Description of the problem and suggested fix: I think it is worth mentioning, that a tuple type (in this case (key, value)) is used as a loop variable to iterate over hash maps.