[Doc] Fix Docstring in extern.py for Sphinx (apache#16482)

Fix a missing line in the docstring of `extern.py`, which would cause
Sphinx to fail to build the documentation.

python/tvm/relax/frontend/nn/extern.py

@@ -138,6 +138,7 @@ def shape_dtype_inference(a, b):
     To expose the symbol, `TVM_DLL_EXPORT_TYPED_FUNC(symbol, function)` is guaranteed available:
 
     .. code-block:: C++
+
         // those headers are guaranteed to be available
         #include <dlpack/dlpack.h>
         #include <tvm/runtime/data_type.h>
@@ -152,7 +153,6 @@ def shape_dtype_inference(a, b):
         // expose symbol `my_func` instead of `_my_func_impl`
         TVM_DLL_EXPORT_TYPED_FUNC(my_func, _my_func_impl);
 
-
     **A compiler pass `AttachExternModules`.** It is introduced to attach a list of
     `nn.ExternModule`s into an IRModule at any stage of the compilation pipeline,
     and attach the compiled external modules as `runtime.Module`s into IRModule's `external_mods`