Fixed an error in the docstring of system.py
Fixed an error in the docstring of system.py

I needed to pass in the coordinates and speeds as lists instead of sympy
symbols in the example in SymbolicSystem's docstring.

yashsharan/sympy,aktech/sympy,chaffra/sympy,jaimahajan1997/sympy,kaushik94/sympy,sampadsaha5/sympy,jaimahajan1997/sympy,skidzo/sympy,souravsingh/sympy,kaushik94/sympy,kaushik94/sympy,souravsingh/sympy,yashsharan/sympy,postvakje/sympy,skidzo/sympy,sampadsaha5/sympy,sampadsaha5/sympy,postvakje/sympy,aktech/sympy,postvakje/sympy,chaffra/sympy,madan96/sympy,souravsingh/sympy,jaimahajan1997/sympy,madan96/sympy,skidzo/sympy,yashsharan/sympy,chaffra/sympy,aktech/sympy,madan96/sympy