Update users.php

I was getting an error because of session_start(), it might just be my version. I read that to fix the error, session_start() should be at the beginning.  Here's the error I got:
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\server\apache\htdocs\3750-master\header.php:9) in C:\server\apache\htdocs\3750-master\classes\users.php on line 29