Update README.md

Author: jbnsn

README.md

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 The file [`main.py`](main.py) contains a most simple implementation of [Kitzes (2013)](https://doi.org/10.3390/resources2040489) in Python with the packages [NumPy](https://numpy.org/) and [Pandas](https://pandas.pydata.org/).
 
-Kitzes, Justin. 2013. ‘An Introduction to Environmentally-Extended Input-Output Analysis’. Resources 2 (4): 489–503. [https://doi.org/10.3390/resources2040489](https://doi.org/10.3390/resources2040489).
+> Kitzes, Justin. 2013. ‘An Introduction to Environmentally-Extended Input-Output Analysis’. Resources 2 (4): 489–503. [https://doi.org/10.3390/resources2040489](https://doi.org/10.3390/resources2040489).
 
 Output of [`main.py`](main.py):
 ```Txt