Update modelingtoolkit.md

Author: ChrisRackauckas

docs/src/API/modelingtoolkit.md

@@ -16,4 +16,4 @@ Secondly, one can generate `OptimizationProblem`s for use in
 Optimization.jl from purely a symbolic front-end. This is the form
 users will encounter when using ModelingToolkit.jl directly, and it is
 also the form supplied by domain-specific languages. For more information,
-see the [OptimizationSystem documentation](https://docs.sciml.ai/ModelingToolkit/stable/systems/OptimizationSystem/).
+see the [OptimizationSystem documentation](https://docs.sciml.ai/ModelingToolkit/stable/API/problems/#SciMLBase.OptimizationProblem).