Incorrect documentation for `write!` in `std::fmt`

The [`std::fmt` docs](https://doc.rust-lang.org/std/fmt/index.html#write) currently say about the `write!` macro:

> Under the hood, this function is actually invoking the `write` function defined in this module.

But in fact, `write!` is defined as calling the `write_fmt` method on its argument, as per [its documentation](https://doc.rust-lang.org/std/macro.write!.html).
