Update refactorization.md (#1017)

doc/fluid/design/motivation/refactorization.md

@@ -121,29 +121,28 @@ Compile Time -> IR -> Runtime
     ```
   - Automatic Model Parallelism (planned for future)
 
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 ## Operator/OpWithKernel/OpKernel
 
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-## Operator
+
+### Operator
 
 * `Operator` is the fundamental building block of the user interface.
     * Operator stores input/output variable names and attributes.
     * The `InferShape` interface is used to infer the shape of the output variables based on the shapes of the input variables.
     * Use `Run` to compute the `output` variables from the `input` variables.
 
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-## OpWithKernel/Kernel
+
+### OpWithKernel/Kernel
 
 * `OpWithKernel` inherits `Operator`.
 * `OpWithKernel` contains a Kernel map.
     * `OpWithKernel::Run` get device's kernel, and invoke `OpKernel::Compute`.
     * `OpKernelKey` is the map key. Only device place now, but may be data type later.
 
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+
 
 ## Why separate Kernel and Operator