Update README.md

Author: joelgrus

README.md

@@ -38,7 +38,7 @@ and so on and so forth.
 18. [Neural Networks](https://github.com/joelgrus/data-science-from-scratch/blob/master/scratch/neural_networks.py)
 19. [Deep Learning]
 20. [Clustering](https://github.com/joelgrus/data-science-from-scratch/blob/master/scratch/clustering.py)
-21. [Natural Language Processing](https://github.com/joelgrus/data-science-from-scratch/blob/master/scratch/natural_language_processing.py)
+21. [Natural Language Processing](https://github.com/joelgrus/data-science-from-scratch/blob/master/scratch/nlp.py)
 22. [Network Analysis](https://github.com/joelgrus/data-science-from-scratch/blob/master/scratch/network_analysis.py)
 23. [Recommender Systems](https://github.com/joelgrus/data-science-from-scratch/blob/master/scratch/recommender_systems.py)
 24. [Databases and SQL](https://github.com/joelgrus/data-science-from-scratch/blob/master/scratch/databases.py)