remove references to PTB perplexity numbers

removing them until we can give appropriate command-line usage that allows users to try the script on PTB again

Author: soumith

word_language_model/README.md

@@ -45,12 +45,8 @@ With these arguments, a variety of models can be tested.
 As an example, the following arguments produce slower but better models:
 
 ```bash
-python main.py --cuda --emsize 650 --nhid 650 --dropout 0.5 --epochs 40           # Test perplexity of 80.97
-python main.py --cuda --emsize 650 --nhid 650 --dropout 0.5 --epochs 40 --tied    # Test perplexity of 75.96
-python main.py --cuda --emsize 1500 --nhid 1500 --dropout 0.65 --epochs 40        # Test perplexity of 77.42
-python main.py --cuda --emsize 1500 --nhid 1500 --dropout 0.65 --epochs 40 --tied # Test perplexity of 72.30
+python main.py --cuda --emsize 650 --nhid 650 --dropout 0.5 --epochs 40           
+python main.py --cuda --emsize 650 --nhid 650 --dropout 0.5 --epochs 40 --tied    
+python main.py --cuda --emsize 1500 --nhid 1500 --dropout 0.65 --epochs 40        
+python main.py --cuda --emsize 1500 --nhid 1500 --dropout 0.65 --epochs 40 --tied 
 ```
-
-Perplexities on PTB are equal or better than
-[Recurrent Neural Network Regularization (Zaremba et al. 2014)](https://arxiv.org/pdf/1409.2329.pdf)
-and are similar to [Using the Output Embedding to Improve Language Models (Press & Wolf 2016](https://arxiv.org/abs/1608.05859) and [Tying Word Vectors and Word Classifiers: A Loss Framework for Language Modeling (Inan et al. 2016)](https://arxiv.org/pdf/1611.01462.pdf), though both of these papers have improved perplexities by using a form of recurrent dropout [(variational dropout)](http://papers.nips.cc/paper/6241-a-theoretically-grounded-application-of-dropout-in-recurrent-neural-networks).