Fix docs for OsStr

[whentze]

At present, there are two small issues with the docs for std::ffi::OsStr:

• The docs say "OsStr is to OsString as String is to &str: the former in each pair are borrowed references; the latter are owned strings.". The latter pair is mixed up: String is the owned variant whereas &str is the borrowed reference.
• The doc links to String and &str are broken and render as [String] and [&str].

This PR fixes these issues.

[rust-highfive]

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

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[Mark-Simulacrum]

r? @GuillaumeGomez

[Diggsey]

This should either be &OsStr and &str or OsStr and str

[GuillaumeGomez]

All good except @Diggsey's comment. Please fix it and then r=me.

[whentze]

r? @GuillaumeGomez

[GuillaumeGomez]

Thanks!

@bors: r+ rollup

[bors]

📌 Commit 8002876 has been approved by GuillaumeGomez