add types to string bindings

rust/book/src/main.rs

    }
    // strings
    let string_slice = "Hello, World!";
    let mut string = string_slice.to_string();
    let string_slice: &str = "Hello, World!";
    let mut string: String = string_slice.to_string();
    println!("{} = {}", string_slice, string);
    string.push_str(" (again...)");