fix quote

Author: cbhyphen

site/en/guide/migrate/early_stopping.ipynb

@@ -457,7 +457,7 @@
         "epochs = 100\n",
         "patience = 5\n",
         "wait = 0\n",
-        "best = float("inf")\n",
+        "best = float('inf')\n",
         "\n",
         "for epoch in range(epochs):\n",
         "    print(\"\\nStart of epoch %d\" % (epoch,))\n",