Fix documentation link to codec module/crate

[marcospb19]

I wanted to use the counterpart tokio_util::io::SinkWriter but there's none, instead the docs point to "codec" but the link is broken.

Codec link can be found here:

https://docs.rs/tokio-util/latest/tokio_util/io/struct.SinkWriter.html

Or here:

tokio/tokio-util/src/io/sink_writer.rs

Lines 26 to 27 in 910a1e2

	/// The opposite conversion of implementing `Sink<_>` for an [`AsyncWrite`]
	/// is done using the [`codec`] module.

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I believe the link could point to tokio_codec::BytesCodec instead, but I'm not sure.

[Darksonn]

To fix this, replace tokio_util with crate in the link.