Correct variable used in pickle dump in `mind_utils.ipynb`

Fixed an issue where the incorrect variable `word_dict` was being dumped to word_dict_all.pkl instead of `word_dict_all` in the `mind_utils.ipynb` notebook.

examples/01_prepare_data/mind_utils.ipynb

@@ -306,7 +306,7 @@
                 "    pickle.dump(word_dict, f)\n",
                 "    \n",
                 "with open(os.path.join(output_path, 'word_dict_all.pkl'), 'wb') as f:\n",
-                "    pickle.dump(word_dict, f)"
+                "    pickle.dump(word_dict_all, f)"
             ]
         },
         {