Update README.md

Author: kimhc6028

README.md

@@ -23,7 +23,7 @@ If you want to do this with very deep model (20 layers),
 
 	$ python forward_thinking.py --epoch 200 --deep
 
-As a comparison, learn by back propagation:
+As a comparison, learn by backpropagation:
 
 	$ python backpropagate.py