Add example in AsMut trait documentation

[Mark-Simulacrum]

Let me know of any changes I should make.

r? @GuillaumeGomez

[rust-highfive]

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

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[GuillaumeGomez]

We generally put examples at the end of the doc. Please move it and then I'll r+.

[Mark-Simulacrum]

Should I move the other examples in the same file? I put the example in the same place as in the AsRef documentation.

[GuillaumeGomez]

No, don't bother. Thanks!

@bors: r+ rollup

[bors]

📌 Commit 102ee5e has been approved by GuillaumeGomez