updated birds dataset download source (#1593)

Author: chenonit

introduction_to_amazon_algorithms/object_detection_birds/object_detection_birds.ipynb

@@ -152,7 +152,11 @@
    "outputs": [],
    "source": [
     "%%time\n",
-    "download('http://www.vision.caltech.edu/visipedia-data/CUB-200-2011/CUB_200_2011.tgz')"
+    "# download('http://www.vision.caltech.edu/visipedia-data/CUB-200-2011/CUB_200_2011.tgz')\n",
+    "# CalTech's download is (at least temporarily) unavailable since August 2020.\n",
+    " \n",
+    "# Can now use one made available by fast.ai .\n",
+    "download ('https://s3.amazonaws.com/fast-ai-imageclas/CUB_200_2011.tgz')"
    ]
   },
   {
@@ -1221,4 +1225,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 2
-}
+}