Update reference link in dpll.py (old link broken)
Update reference link in dpll.py (old link broken)

jaimahajan1997/sympy,souravsingh/sympy,ChristinaZografou/sympy,sampadsaha5/sympy,yashsharan/sympy,rahuldan/sympy,skidzo/sympy,kaushik94/sympy,rahuldan/sympy,jaimahajan1997/sympy,drufat/sympy,kaushik94/sympy,madan96/sympy,skidzo/sympy,hargup/sympy,souravsingh/sympy,jaimahajan1997/sympy,hargup/sympy,ChristinaZografou/sympy,madan96/sympy,chaffra/sympy,aktech/sympy,chaffra/sympy,drufat/sympy,rahuldan/sympy,aktech/sympy,skidzo/sympy,sampadsaha5/sympy,yashsharan/sympy,postvakje/sympy,chaffra/sympy,sampadsaha5/sympy,yashsharan/sympy,aktech/sympy,postvakje/sympy,postvakje/sympy,hargup/sympy,drufat/sympy,kaushik94/sympy,madan96/sympy,souravsingh/sympy,ChristinaZografou/sympy