Fix example in the docs

[andrew-d]

Relevant to #4140

For example, this fails to compile:

trait Printable {
  fn to_str() -> ~str;
}

impl ~str: Printable {
  fn to_str() -> ~str { copy self }
}

fn main() {
    let s = ~"foobar";
    let v = s.to_str();
    assert v == s;
}

But this passes:

trait Printable {
  fn make_string() -> ~str;
}

impl ~str: Printable {
  fn make_string() -> ~str { copy self }
}

fn main() {
    let s = ~"foobar";
    let v = s.make_string();
    assert v == s;
}

[andrew-d]

Whoops, wrong branch. Closing and re-opening.