add matrix-multiplication example to notebook

100 Numpy exercises.ipynb

@@ -477,7 +477,10 @@
    "outputs": [],
    "source": [
     "Z = np.dot(np.ones((5,3)), np.ones((3,2)))\n",
-    "print(Z)"
+    "print(Z)",
+    "\n\n",
+    "# Alternative solution, in Python 3.5 and above\n",
+    "Z = np.ones((5,3)) @ np.ones((3,2))"
    ]
   },
   {