add @oyamad suggestion to mention the use of vdot

Author: mmcky

lectures/opt_transport.md

@@ -685,6 +685,8 @@ total_cost = np.vdot(X, C)
 total_cost
 ```
 
+Here we use [np.vdot](https://numpy.org/doc/stable/reference/generated/numpy.vdot.html) for the trace inner product of X and C
+
 ### A Larger Application
 
 Now let's try using the same package on a slightly larger application.