Fix stdin.rs comments to say "read" instead of "write".

[sunfishcode]

This just fixes a few comments that appear to have been copied and
pasted from stdout.rs.

[sunfishcode]

The CI failure is unrelated:

error: unknown `doc` attribute `unstable`
   --> src/utils.rs:145:46
    |
145 |               #[cfg_attr(feature = "docs", doc(unstable))]
    |                                                ^^^^^^^^
    | 
   ::: src/lib.rs:336:1
    |
336 | / cfg_unstable_default! {
337 | |     #[doc(inline)]
338 | |     pub use std::{write, writeln};
339 | | }
    | |_- in this macro invocation

[sunfishcode]

Rebased, and now the CI is green.

[Fishrock123]

I believe this is correct, though some stdin pipes can also be written to but I don't think that async-std supports that.