added back missing file package import [(#2150)](GoogleCloudPlatform/python-docs-samples#2150)

Author: merla18

samples/tables/notebooks/energy_price_forecasting/energy_price_forecasting.ipynb

@@ -202,6 +202,7 @@
       "source": [
         "#@title Authenticate using service account key and create a client. { vertical-output: true }\n",
         "\n",
+        "from google.colab import files\n",
         "from google.cloud import automl_v1beta1\n",
         "\n",
         "# Upload service account key\n",