fix typo in README

Author: ndem0

README.md

@@ -104,7 +104,7 @@ After the training we can infer our model, save it or just plot the PINN approxi
 
 
 ## Dependencies and installation
-**PINA** requires requires `numpy`, `scipy`, `matplotlib`, `future`, `torch`, `sphinx` (for the documentation) and `pytest` (for local test). The code is tested for Python 3, while compatibility of Python 2 is not guaranteed anymore. It can be installed using `pip` or directly from the source code.
+**PINA** requires `numpy`, `scipy`, `matplotlib`, `future`, `torch`, `sphinx` (for the documentation) and `pytest` (for local test). The code is tested for Python 3, while compatibility of Python 2 is not guaranteed anymore. It can be installed using `pip` or directly from the source code.
 
 ### Installing via PIP
 Mac and Linux users can install pre-built binary packages using pip.