change wrong variable name

Author: ArnoStrouwen

docs/src/highlevel.md

@@ -308,7 +308,7 @@ sys = modelingtoolkitize(prob)
 Using this, we can symbolically build the Jacobian and then rebuild the ODEProblem:
 
 ```julia
-jac = eval(ModelingToolkit.generate_jacobian(de...)[2])
+jac = eval(ModelingToolkit.generate_jacobian(sys...)[2])
 f = ODEFunction(rober, jac=jac)
 prob_jac = ODEProblem(f,[1.0,0.0,0.0],(0.0,1e5),(0.04,3e7,1e4))
 ```