Fix use of unaligned references in example

[adamgreig]

A recent change in Rust to deny unaligned reference by default (https://github.com/rust-lang/rust/pull/95372) caused the upstream rust build of this book to break: https://github.com/rust-lang/rust/pull/96108#issuecomment-1100646376

This PR removes the unaligned references, instead using addr_of!() to get a raw pointer to print.

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[bors[bot]]

Build succeeded:

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