update cpp_saving_tutorial.rst
update cpp_saving_tutorial.rst


Former-commit-id: 50c3be5fa3c9eef8b2a512e5cf1f6fe8b4646ac4
danielhers/dynet,clab/cnn,clab/cnn,danielhers/dynet,chunyang-wen/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,clab/cnn,clab/dynet,xunzhang/dynet,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,clab/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet