Replace np.sum(a * b) with np.vdot(a, b)

Author: suda-yuga

lectures/opt_transport.md

@@ -681,7 +681,7 @@ X
 Sure enough, we have the same solution and the same cost
 
 ```{code-cell} ipython3
-total_cost = np.sum(X * C)
+total_cost = np.vdot(X, C)
 total_cost
 ```