fix(BERT): fix doc bug

Change-Id: I8ead5f793bf4bbc721f7efd00569f1e01789abe0
(cherry picked from commit 6f591f1d76d3e7c1214b46a63cafa7c069efc859)

Author: siyuan.yang

sample/BERT/docs/BERT4torch_Exportonnx_Guide.md

@@ -1,7 +1,8 @@
 # bert4torch模型onnx导出
 bert4torch模型导出是在Pytorch模型的生产环境下进行的，需提前根据[A simple training framework that recreates bert4keras in PyTorch. bert4torch](https://github.com/Tongjilibo/bert4torch/)的要求安装好Pytorch环境，准备好相应的代码和模型，并保证模型能够在Pytorch环境下正常推理运行。
+
 torch==1.13.0
-'''
+```python
 import torch
 import torchvision
 
@@ -10,4 +11,4 @@ input_data = torch.zeros([1, 256]) # generate input data
 output_onnx_path = 'test.onnx'
 
 torch.onnx.export(model, input_data, output_onnx_path, verbose=True, opset_version=12)
-'''
+```