Typo in the `vanstiff` docstring

[Nathanael Bosch]

The ODEs in the docstrings of prob_ode_vanderpol and prob_ode_vanstiff are not the same, even though both use the same van function.

If the problem is correctly defined, then line 83

DiffEqProblemLibrary.jl/src/ode/ode_simple_nonlinear_prob.jl

Line 83 in 87ca6f6

 \\frac{dy}{dt} = μ(1-x^2)y -x 

should be changed to be the same as line 67

DiffEqProblemLibrary.jl/src/ode/ode_simple_nonlinear_prob.jl

Line 67 in 87ca6f6

 \\frac{dy}{dt} = μ((1-x^2)y -x) 

I'll open a PR right away.