Improve fill-mask pipeline example in 03-pipelines notebook.

Remove hardcoded mask_token and use the value provided by the tokenizer.

notebooks/03-pipelines.ipynb

@@ -294,7 +294,7 @@
    ],
    "source": [
     "nlp_fill = pipeline('fill-mask')\n",
-    "nlp_fill('Hugging Face is a French company based in <mask>')"
+    "nlp_fill('Hugging Face is a French company based in ' + nlp_fill.tokenizer.mask_token)"
    ]
   },
   {