Fix type hint to match php docs

From http://php.net/manual/en/function.posix-isatty.php

bool posix_isatty ( mixed $fd )

Determines if the file descriptor fd refers to a valid terminal type device.

fd

    The file descriptor, which is expected to be either a file
    resource or an integer. An integer will be assumed to be a file
    descriptor that can be passed directly to the underlying system
    call.

    In almost all cases, you will want to provide a file resource.

Change-Id: Ief19ac1f09e0a955c8f5d5af28b87b8e8d427a2e