Update bidirectional_rnn.ipynb (#380)

Replace broken link for Sepp Hochreiter & Jurgen Schmidhuber's LSTM document.

notebooks/3_NeuralNetworks/bidirectional_rnn.ipynb

@@ -23,7 +23,7 @@
     "<img src=\"https://ai2-s2-public.s3.amazonaws.com/figures/2016-11-08/191dd7df9cb91ac22f56ed0dfa4a5651e8767a51/1-Figure2-1.png\" alt=\"nn\" style=\"width: 600px;\"/>\n",
     "\n",
     "References:\n",
-    "- [Long Short Term Memory](http://deeplearning.cs.cmu.edu/pdfs/Hochreiter97_lstm.pdf), Sepp Hochreiter & Jurgen Schmidhuber, Neural Computation 9(8): 1735-1780, 1997.\n",
+    "- [Long Short Term Memory](https://www.researchgate.net/profile/Sepp_Hochreiter/publication/13853244_Long_Short-term_Memory/links/5700e75608aea6b7746a0624/Long-Short-term-Memory.pdf), Sepp Hochreiter & Jurgen Schmidhuber, Neural Computation 9(8): 1735-1780, 1997.\n",
     "\n",
     "## MNIST Dataset Overview\n",
     "\n",