fix contract python doc
fix contract python doc

Former-commit-id: 9d09989f881202d1e2be7167c9999a33bbecb810
xunzhang/dynet,danielhers/dynet,danielhers/dynet,shuheik/dynet,clab/cnn,xunzhang/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,danielhers/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,danielhers/dynet,clab/cnn,chunyang-wen/dynet,clab/dynet,clab/dynet