Fix a doctest

Author: kwankyu

src/doc/en/a_tour_of_sage/index.rst

@@ -125,7 +125,6 @@ This asks Sage to factor a polynomial in two variables.
      x^51*y^9 - x^48*y^12 + x^42*y^18 + x^39*y^21 - x^33*y^27 -
      x^30*y^30 - x^27*y^33 + x^21*y^39 + x^18*y^42 - x^12*y^48 -
      x^9*y^51 + x^3*y^57 + y^60)
-
     sage: F.expand()
     x^99 + y^99