Update training.ipynb (#66)

Need to `pip install datasets`

Also the first link to the HuggingFace datasets page points to a 404 page

https://huggingface.co/github.com/huggingface/datasets

Author: msaroufim

transformers_doc/training.ipynb

@@ -9,7 +9,8 @@
     "# Transformers installation\n",
     "! pip install transformers\n",
     "# To install from source instead of the last release, comment the command above and uncomment the following one.\n",
-    "# ! pip install git+https://github.com/huggingface/transformers.git\n"
+    "# ! pip install git+https://github.com/huggingface/transformers.git\n",
+    "! pip install datasets\n"
    ]
   },
   {