update readme

README.md

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 A small educational React website to understand how computational graph and derivatives work. I created this tool because I couldn't fully understand how back-propagation works in neural networks.
 
-Classic derivative problem $\frac{\sin{x}}{x}$:
+Classic calculus problem $\frac{\sin{x}}{x}$:
 
 ![screenshot 1](./screenshots/screenshot1.png "Screenshot 1")