Fix SelectServers example.
Fix SelectServers example.

Fixes #160.

deafgoat/mgo,Yottaa/mgo.v2,deafgoat/mgo,hexadecy/mgo,hexadecy/mgo,hexadecy/mgo,deafgoat/mgo,go-mgo/travis-test,gabrielrussell/mgo,DroiTaipei/mgo,DroiTaipei/mgo,Yottaa/mgo.v2,go-mgo/travis-test,deafgoat/mgo,jackspirou/mgo,gabrielrussell/mgo,hexadecy/mgo,Yottaa/mgo.v2,jackspirou/mgo,Yottaa/mgo.v2,go-mgo/travis-test,gabrielrussell/mgo,jackspirou/mgo,gabrielrussell/mgo,jackspirou/mgo,DroiTaipei/mgo,DroiTaipei/mgo,go-mgo/travis-test