Update datebox.po

The original format '%Y-%m-%d' doesn't work with latest version. 

When user pan a date from calbox, the calbox won't disappear and error shows:
Uncaught TypeError: Cannot read property 'length' of undefined

It's probably string handle '-' symbol method incorrectly. Two workarounds: 

1. you can take one more space between %m and %d.
For instances: dateFormat: "%Y-%m -%d"   
For better programmatic, you could take unicode as alternative."%Y-%m\u0020-%d" 

2. Or, either use '/' for delimiter instead.This is what i preferred.
For instances: dateFormat: "%Y/%m/%d"