Fix typo in ASYNC_WAIT_CTX_new.pod doc

For the function that get the changed fds, it should be
'ASYNC_WAIT_CTX_get_changed_fds()' instead of 'ASYNC_WAIT_CTX_fds_have_changed()'.

CLA: trivial

Signed-off-by: Paul Yang <paulyang.inf@gmail.com>

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2967)