Add new example - immutable_borrow_lifetime & Correct wrong example

[c0wjay]

This example illustrate examples below - from The Rust Programming Language Book

fn main() {
    let string1 = String::from("abcd");
    let string2 = String::from("xyz");

    let result = longest(&string1, &string2);
    println!("The longest string is {}", result);
}

fn longest<'a>(x: &'a String, y: &'a String) -> &'a String {
    if x.len() > y.len() {
        x
    } else {
        y
    }
}

[c0wjay]

This PR also includes correction of wrong example in struct_lifetime