docs: fix [n/.] separator example output (12.345.0 → 12.345,0)

[pacocartones]

The README 'Separator' example asserts the wrong output for the [n/.] case:

f = "[n/.]".parse().unwrap(); // use a period
assert_eq!(f.fmt(12345.0), "12.345.0");

Choosing a period as the thousands separator switches the decimal marker to a comma, so the current release produces "12.345,0", not "12.345.0". This is confirmed by the crate's own module doc-comment, which is compiled as a doctest and asserts the correct value for the identical call:

//! f = "[n/.]".parse().unwrap(); // use period and commas
//! assert_eq!(f.fmt(12345.0), "12.345,0");

The //! doctest was updated when the behaviour changed but the README example was not (the README is not run as a doctest). This one-line fix aligns the README with the tested behaviour and clarifies the comment. No code change.

[pacocartones]

Withdrawing this — on reflection the repo's low recent activity makes a doc nitpick like this not worth your review time. Apologies for the noise.