Update README.md

Author: FilippoMB

README.md

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 This code reproduces the experimental results obtained with the MinCutPool layer 
 as presented in the ICML 2020 paper 
 
-[Spectral Clustering with Graph Neural Networks for Graph Pooling](https://arxiv.org/abs/1907.00481)  
+[Spectral Clustering with Graph Neural Networks for Graph Pooling](https://arxiv.org/pdf/1907.00481.pdf)  
 F. M. Bianchi*, D. Grattarola*, C. Alippi
 
 The official implementation of the MinCutPool layer can be found in