Fix identifier example in top-level docs

[tchajed]

Formerly this example wouldn't parse single-letter identifiers, which might confuse someone using it directly.

If you'd like, another possible improvement is to make the doc test run and add the following to main:

fn main() {
    doc::identifier("a").expect("a");
    doc::identifier("foo1").expect("foo1");
    doc::identifier("1up").expect_err("starts with number");
    doc::identifier("hello there").expect_err("has a space");
}

[kevinmehall]

Good find, thanks!