added Sutton book's equation

Author: Aerin Kim

DP/Value Iteration Solution.ipynb

@@ -74,7 +74,7 @@
     "            best_action_value = np.max(A)\n",
     "            # Calculate delta across all states seen so far\n",
     "            delta = max(delta, np.abs(best_action_value - V[s]))\n",
-    "            # Update the value function\n",
+    "            # Update the value function. Ref: Sutton book eq. 4.10. \n",
     "            V[s] = best_action_value        \n",
     "        # Check if we can stop \n",
     "        if delta < theta:\n",