Fix NameError in visualizing calibration metrics tutorial (quantumlib#4316)

`cirq_google` is used to get calibrations in line 181 but never imported, leading to `NameError: name cirq_google is not defined` (189945709).

Author: rmlarose

docs/tutorials/google/visualizing_calibration_metrics.ipynb

@@ -80,7 +80,8 @@
     "    print(\"installing cirq...\")\n",
     "    !pip install --quiet cirq\n",
     "    print(\"installed cirq.\")\n",
-    "    import cirq\n",
+    "import cirq\n",
+    "import cirq_google\n",
     "import os\n",
     "import matplotlib.pyplot as plt"
    ]