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