Simplify the unsafe slice construction example

[haranishi]

Use a single four-element u16 array to demonstrate the bytes-versus-elements mistake in slice::from_raw_parts. Removing KeyPair, repr(C), and the adjacent private-key array keeps the example focused on the unsafe function's safety requirements. The speaker notes identify the length mismatch and give a live edit that restores the correct slice length.

The starting example intentionally remains unsound, as required by the lesson.

Fixes #2733.

Validation:

• Miri with nightly-2025-09-01 reports the expected undefined behavior: the slice requires 16 bytes but the array provides 8.
• Applying the speaker-note edit (four elements) passes Miri and prints [1, 2, 3, 4].
• mdbook build, including the local link check.
• mdbook serve: the changed page returns HTTP 200 with the simplified example and updated notes.
• dprint check src/unsafe-rust/unsafe-functions/calling.md and git diff --check.

Local setup: cargo xtask install-tools hit a thiserror_impl E0463 error in Bazel on macOS. The repository tool versions were installed with Cargo for the checks above.

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