message: [doc] Fix dead link to custom type mapping
diff: @@ -211,8 +211,9 @@ including a definition of another alias:
  using times = array<time>;
  ```
  -Type aliases can optionally be [mapped to custom types](#custom-type-mappings)
  -in the generated code.
  +Type aliases can optionally be mapped to custom types in the generated code
  +for both [C++](bond_cpp.html#custom-type-mappings) and
  +[C#](bond_cs.html#custom-type-mappings).
  See examples:
  