Broken link on readme.md

[Soumee Mukherjee]

Link for the University of Pennsylvania research group under prominent NLP research groups seems to be broken

[Jean-Yves Guyomarc'h]

@SOUMEE2000 it might have been fixed in the meantime. Both are working for me:

• https://nlp.cis.upenn.edu/
• https://www.seas.upenn.edu/~pdtb/