fix documentation for date picker - alt date format

The documentation for the date picker alternate date format is incorrect. If you go to

http://jqueryui.com/demos/datepicker/#option-altFormat

and click on options and then click on altFormat the code examples say altFormat instead of dateFormat

For example

$('.selector').datepicker({ altFormat: 'yy-mm-dd' });

should be changed to

$('.selector').datepicker({ dateFormat: 'yy-mm-dd' });