io: remove `PollEvented` docs reference to `clear_read_ready` (#4931)
io: remove `PollEvented` docs reference to `clear_read_ready` (#4931)

The documentation for`PollEvented` had a single remaining reference to
`Registration::clear_read_ready`, which no longer exists. This change
updates the documentation to refer to `Registration::clear_readiness`
instead.
tokio-rs/tokio,tokio-rs/tokio