Rollup merge of rust-lang#50624 - adevore:fs-write-str-example, r=steveklabnik

fs::write: Add example writing a &str

This adds an example to the documentation for `fs::write` that demonstrates the use of the `AsRef<[u8]>` implementation for &str. Experienced users should already recognize the possibility from the type signature, but new users may not recognize the significance.

Author: kennytm