add solution with matrix-mult operator

This solution work for Python 3.5 and above.

100 Numpy exercises.md

@@ -208,6 +208,10 @@ color = np.dtype([("r", np.ubyte, 1),
 ```python
 Z = np.dot(np.ones((5,3)), np.ones((3,2)))
 print(Z)
+
+# Alternative solution, in Python 3.5 and above
+Z = np.ones((5,3)) @ np.ones((3,2))
+print(Z)
 ```
 
 #### 25. Given a 1D array, negate all elements which are between 3 and 8, in place. (★☆☆)