updated test utils import in d2go_beginner.ipynb

Summary:
In 9d238344, the test utils were moved to the core library, but the import for the create_fake_detection_data_loader inside the d2go_beginner.ipynb wasn't updated.

Pull Request resolved: https://github.com/facebookresearch/d2go/pull/29

Reviewed By: newstzpz

Differential Revision: D27239846

Pulled By: zhanghang1989

fbshipit-source-id: e39df32746b1d1081026f9969bda84e73ac7df55

demo/d2go_beginner.ipynb

@@ -580,7 +580,7 @@
     "import copy\n",
     "from detectron2.data import build_detection_test_loader\n",
     "from d2go.export.api import convert_and_export_predictor\n",
-    "from d2go.tests.data_loader_helper import create_fake_detection_data_loader\n",
+    "from d2go.utils.testing.data_loader_helper import create_fake_detection_data_loader\n",
     "from d2go.export.d2_meta_arch import patch_d2_meta_arch\n",
     "\n",
     "import logging\n",