message: fix broken section links in the document
diff: @@ -451,17 +451,17 @@ Now that you know how to build a basic (and somewhat restricted) op and
  implementation, we'll look at some of the more complicated things you will
  typically need to build into your op. This includes:
  -* [Conditional checks and validation](#conditional_checks_and_validation)
  -* [Op registration](#op_registration)
  +* [Conditional checks and validation](#conditional-checks-and-validation)
  +* [Op registration](#op-registration)
  * [Attrs](#attrs)
  - * [Attr types](#attr_types)
  + * [Attr types](#attr-types)
  * [Polymorphism](#polymorphism)
  - * [Inputs and outputs](#inputs_and_outputs)
  - * [Backwards compatibility](#backwards_compatibility)
  -* [GPU support](#gpu_support)
  - * [Compiling the kernel for the GPU device](#compiling_the_kernel_for_the_gpu_device)
  -* [Implement the gradient in Python](#implement_the_gradient_in_python)
  -* [Shape functions in C++](#shape_functions_in_c)
  + * [Inputs and outputs](#inputs-and-outputs)
  + * [Backwards compatibility](#backwards-compatibility)
  +* [GPU support](#gpu-support)
  + * [Compiling the kernel for the GPU device](#compiling-the-kernel-for-the-gpu-device)
  +* [Implement the gradient in Python](#implement-the-gradient-in-python)
  +* [Shape functions in C++](#shape-functions-in-c)
  ### Conditional checks and validation
  