Fix the missing line in the guessing-game.md

[vessd]

r? @steveklabnik

[rust-highfive]

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

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[steveklabnik]

@bors: r+ rollup

thank you! That's... odd. I wonder how it happened.

[bors]

📌 Commit 6a6e9a9 has been approved by steveklabnik