Change to use virtual engine in cirq_google docs (quantumlib#5858)

- Change to use noisy virtual engine in tutorial docs
so that doc infra can run.  These are not linked in the navigation
anyway.

Author: dstrain115

docs/tutorials/google/colab.ipynb

@@ -179,7 +179,8 @@
     "processor_id = \"\" #@param {type:\"string\"}\n",
     "\n",
     "from cirq_google.engine.qcs_notebook import get_qcs_objects_for_notebook\n",
-    "qcs_objects = get_qcs_objects_for_notebook(project_id, processor_id)\n",
+    "# For real engine instances, delete 'virtual=True' below.\n",
+    "qcs_objects = get_qcs_objects_for_notebook(project_id, processor_id, virtual=True)\n",
     "engine = qcs_objects.engine\n",
     "processor_id = qcs_objects.processor_id"
    ]

docs/tutorials/google/start.ipynb

@@ -142,7 +142,8 @@
     "processor_id = \"\" #@param {type:\"string\"}\n",
     "\n",
     "from cirq_google.engine.qcs_notebook import get_qcs_objects_for_notebook\n",
-    "qcs_objects = get_qcs_objects_for_notebook(project_id, processor_id)\n",
+    "# For real engine instances, delete 'virtual=True' below.\n",
+    "qcs_objects = get_qcs_objects_for_notebook(project_id, processor_id, virtual=True)\n",
     "\n",
     "project_id = qcs_objects.project_id\n",
     "processor_id = qcs_objects.processor_id\n",