Fix broken link to structs in Polymorphism section (#48)

Author: bneumann

src/language/polymorphism.md

@@ -11,7 +11,7 @@ See also:
 - [Inheritance][inheritance-section]
 - [Operator overloading][operator-overloading-section]
 
-[structures-section]: ./custom-types.md#structures-struct
+[structures-section]: ./custom-types/structs.md
 [generics-section]: ./generics.md
 [inheritance-section]: ./inheritance.md
 [operator-overloading-section]: ./operator-overloading.md