added back missing file import [(#2145)](GoogleCloudPlatform/python-docs-samples#2145)

Author: merla18

samples/tables/notebooks/census_income_prediction/getting_started_notebook.ipynb

@@ -128,6 +128,7 @@
    "source": [
     "#@title Authenticate 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",