Fix broken link and typo docstring

Author: ArnoStrouwen

docs/src/mtkitize_tutorials/modelingtoolkitize_index_reduction.md

@@ -52,7 +52,7 @@ In this tutorial we will look at the pendulum system:
 ```
 
 As a good DifferentialEquations.jl user, one would follow
-[the mass matrix DAE tutorial](https://diffeq.sciml.ai/stable/tutorials/advanced_ode_example/#Handling-Mass-Matrices)
+[the mass matrix DAE tutorial](https://diffeq.sciml.ai/stable/tutorials/dae_example/#Mass-Matrix-Differential-Algebraic-Equations-(DAEs)
 to arrive at code for simulating the model:
 
 ```@example indexred

src/structural_transformation/codegen.jl

@@ -496,7 +496,7 @@ ODAEProblem(args...; kw...) = ODAEProblem{true}(args...; kw...)
 
 This constructor acts similar to the one for [`ODEProblem`](@ref) with the following changes:
 `ODESystem`s can sometimes be further reduced if `structural_simplify` has
-already been applied to them. This is done this constructor.
+already been applied to them.
 In these cases, the constructor uses the knowledge of the strongly connected
 components calculated during the process of simplification as the basis for
 building pre-simplified nonlinear systems in the implicit solving.