remove "main" from plot_snakes example.
remove "main" from plot_snakes example.

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