Fixed documentation for logistic function
Fixed documentation for logistic function

Abasyoni/dynet,Peilun-Li/dynet,Peilun-Li/dynet,cherryc/dynet,Abasyoni/dynet,cherryc/dynet,cherryc/dynet,mitchellstern/dynet,mitchellstern/dynet,Peilun-Li/dynet,kamigaito/cnn,Abasyoni/dynet,kamigaito/cnn