Fix the manpage example

[hankbao]

I found the output of the manpage example regarding the POLLIN event quite confusing. It ought to be true. But it is false because the pollfd is copied mistakenly.

$ cargo run --example manpage -- 1
...

fault_handler_thread():
    poll() returns: nready = 1; POLLIN = false; POLLERR = false

...

[pchickey]

Thank you for this fix! I had to go review our (retired) use of these functions and we seem to have gotten it right in production, at least, even if the readme has been wrong for a long time.