Fix warning and make code follow the text better
auto merge of #17350 : pablobm/rust/doc-fix, r=steveklabnik

Without this change, this piece of code throws a warning because `std::io` is imported, and then `std::io::stdin` is used explicitly.

From reading the paragraphs that follow that code, I assume that the original intent of the author was to show that code without the first line, the `use` line. That way, the text seems to make better sense to me.