Minor typo fix. GPUS to GPUs.

chapter09_natural-language-processing/tree-lstm.ipynb

@@ -18,7 +18,7 @@
     "[we introduced in chapter 5](../chapter05_recurrent-neural-networks/simple-rnn.ipynb)\n",
     "model word order, by passing over the sequence of words in order,\n",
     "updating the models representation of the sentence after each word. \n",
-    "And, with LSTM recurrent cells and training on GPUS, \n",
+    "And, with LSTM recurrent cells and training on GPUs, \n",
     "even the straightforward LSTM far outpaces classical approaches,\n",
     "on a number of tasks, including language modeling,\n",
     "named entity recognition and more. \n",