DOC: fix typo in example
DOC: fix typo in example

Hooray, my first commit to pystruct!

wattlebird/pystruct,massmutual/pystruct,wattlebird/pystruct,amueller/pystruct,pystruct/pystruct,d-mittal/pystruct,d-mittal/pystruct,massmutual/pystruct,pystruct/pystruct,amueller/pystruct