Auto merge of #51081 - kornelski:examplestr, r=steveklabnik

Auto merge of #51081 - kornelski:examplestr, r=steveklabnik

Use String, not &str in some collection examples

Discussed in #46966

Overuse of borrowed values in data structures is a common mistake I see in Rust user forums. Users who copy&paste such examples end up fighting with the borrow checker as soon as they replace string literals with some real values.

This changes a couple of examples to use `String`, and it adds opportunity to demonstrate use of `Borrow`.