Improve expect messages in fmt docs

[kunwarpadda]

Related issue: #159751

Updates the two user-facing expect messages in
library/core/src/fmt/mod.rs to follow the recommended message style.

Tests:

• ./x test library/core --doc

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) some time within the next two weeks.

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[Kivooeo]

was LLM used for this and if so why?

[apiraino]

@kunwarpadda please disable Copilot in your GitHub settings and don't trigger unprompted reviews from bots in our repository, thanks.

[kunwarpadda]

was LLM used for this and if so why?

Yes, I used an LLM as a quick sanity check to ensure the wording followed the recommended expect message style.

[rust-bors]

☔ The latest upstream changes (presumably #160627) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.