Changed docstring in equation() in order to fix error causing Travis checks to fail
Changed docstring in equation() in order to fix error causing Travis checks to fail

Example:

In [1]: from sympy import Parabola, Point, Line, Symbol
In [2]: p1 = Parabola(Point(0, 0), Line(Point(5, 8), Point(7, 8)))
In [3]: p1.equation()
Out [4]: -x**2 - 16*y + 64
In[5]: p1.equation(Symbol("f"))
Out[6]: -f**2 - 16*y + 64
In[7]: p1.equation(y = Symbol("z"))
Out[8]:  -x**2 - 16*z + 64

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