Clean formatting in `io.rs`

[ojeda]

We don't use @ to refer to parameters or similar in Rust documentation. Also, it is missing a code span.

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This requires submitting a proper patch to the LKML and the Rust for Linux mailing list. Please recall to test your changes (including generating the documentation if changed, running the Rust doctests if changed, etc.), to use a proper title for the commit, to sign your commit under the Developer's Certificate of Origin and to add a Reported-by: tag, a Closes: tag to this issue and a Fixes: tag to the relevant commit. Please see https://docs.kernel.org/process/submitting-patches.html and https://rust-for-linux.com/contributing for details.

[saivishnu725]

Sent a patch