fix doctest (#414)

modified the spacing

Author: jverzani

src/polynomials/LaurentPolynomial.jl

@@ -51,7 +51,7 @@ julia> integrate(q)
 LaurentPolynomial(1.0*x + 0.5*x² + 0.3333333333333333*x³)
 
 julia> integrate(p)  # x⁻¹  term is an issue
-ERROR: ArgumentError: Can't integrate Laurent  polynomial with  `x⁻¹` term
+ERROR: ArgumentError: Can't integrate Laurent polynomial with  `x⁻¹` term
 
 julia> integrate(P([1,1,1], -5))
 LaurentPolynomial(-0.25*x⁻⁴ - 0.3333333333333333*x⁻³ - 0.5*x⁻²)