Update d2go_beginner.ipynb

Summary:
Looks like an import changed, this changes it in the ipynb notebook example.

> Attached screenshot of this working now in collab.

![Screen Shot 2022-06-04 at 11 28 53 AM](https://user-images.githubusercontent.com/8814826/172020883-6501d27d-a917-497c-8ca9-6d956b37dedf.png)

Pull Request resolved: https://github.com/facebookresearch/d2go/pull/276

Reviewed By: zhanghang1989

Differential Revision: D36921926

fbshipit-source-id: e623b61633fb09fc25fa91d23555c62002045933

demo/d2go_beginner.ipynb

@@ -607,7 +607,7 @@
    "source": [
     "import copy\n",
     "from detectron2.data import build_detection_test_loader\n",
-    "from d2go.export.api import convert_and_export_predictor\n",
+    "from d2go.export.exporter import convert_and_export_predictor\n",
     "from d2go.utils.testing.data_loader_helper import create_detection_data_loader_on_toy_dataset\n",
     "from d2go.export.d2_meta_arch import patch_d2_meta_arch\n",
     "\n",