doc: method-syntax.md: method chaining example doesn't show how to return self

[FluffyLlemon]

In the method-syntax.md file:

This is called ‘method chaining’, and we can do it by returning self.

The example (copied below) that immediately follows this does not actually contain code returning self. Instead, a new Circle struct is built and returned.

The wording suggests that the next example is going to show how returning self can be done, but the example doesn't show that.

struct Circle {
    x: f64,
    y: f64,
    radius: f64,
}

impl Circle {
    fn area(&self) -> f64 {
        std::f64::consts::PI * (self.radius * self.radius)
    }

    fn grow(&self, increment: f64) -> Circle {
        Circle { x: self.x, y: self.y, radius: self.radius + increment }
    }
}

fn main() {
    let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
    println!("{}", c.area());

    let d = c.grow(2.0).area();
    println!("{}", d);
}