fix some cross references

Author: ArnoStrouwen

docs/src/examples/higher_order.md

@@ -1,4 +1,4 @@
-# [Automatic Transformation of Nth Order ODEs to 1st Order ODEs](@id higher_order_ODE)
+# [Automatic Transformation of Nth Order ODEs to 1st Order ODEs](@id higher_order)
 
 ModelingToolkit has a system for transformations of mathematical
 systems. These transformations allow for symbolically changing

docs/src/examples/modelingtoolkitize_index_reduction.md

@@ -1,4 +1,4 @@
-# [Automated Index Reduction of DAEs](@ref higher_index)
+# [Automated Index Reduction of DAEs](@id higher_index)
 
 In many cases one may accidentally write down a DAE that is not easily solvable
 by numerical methods. In this tutorial, we will walk through an example of a

docs/src/tutorials/discrete_system.md

@@ -1,4 +1,4 @@
-# (Experimental) [Modeling Discrete Systems](@id discrete)
+# [(Experimental) Modeling Discrete Systems](@id discrete)
 
 In this example, we will use the new [`DiscreteSystem`](@ref) API
 to create an SIR model.

docs/src/tutorials/ode_modeling.md

@@ -390,7 +390,7 @@ If you want some extra modeling goodies:
 If the getting started tutorial is too high-level for you, do a deeper dive:
   -  [Learn everything there is to know about `@mtkmodel` blocks](@ref mtk_language)
   -  [Learn how to write your own components to use in `@mtkmodel`](@ref acausal)
-  -  [Learn how to script the creation of `ODESystems` without the `@mtkmodel`](@id programmatically)
+  -  [Learn how to script the creation of `ODESystems` without the `@mtkmodel`](@ref programmatically)
   -  [Learn how to conserve certain quantities when connecting components using a domain](@ref domains)
   -  [Learn how MTK ensures that the initial state of your model is correct](@ref initialization)
   -  [Learn how MTK reduces the size of your model](@ref tearing)
@@ -410,4 +410,4 @@ Still want to know more about MTK?
   - Not sure how MTK relates to similar tools and packages? Read
     [Comparison of ModelingToolkit vs Equation-Based and Block Modeling Languages](@ref comparison)
   - Want to become a MTK dev? Read
-    [Internal Details](@ref internal)
+    [Internal Details](@ref internals)