Invalid URL in code comment

[atouchet]

futures-rs/futures-util/src/io/mod.rs

Lines 33 to 34 in 4fe6c20

	// used by `BufReader` and `BufWriter`
	// https://github.com/rust-lang/rust/blob/master/src/libstd/sys_common/io.rs#L1

Going to the URL here returns "404 - page not found".