Fix links to english docs

doc/v2/howto/index_en.rst

@@ -6,32 +6,32 @@ PaddlePaddle provides the users the ability to flexibly set various command line
 ..  toctree::
   :maxdepth: 1
 
-  cmd_parameter/index_cn.rst
+  cmd_parameter/index_en.rst
 
 PaddlePaddle supports distributed training tasks on fabric clusters, MPI clusters, and Kubernetes clusters. For detailed configuration and usage instructions, refer to:
 
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   :maxdepth: 1
 
-  cluster/index_cn.rst
+  cluster/index_en.rst
 
 PaddlePaddle provides a C-API for inference. We provide the following guidelines  for using the C-API:
 
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   :maxdepth: 1
 
-  capi/index_cn.rst
+  capi/index_en.rst
 
 PaddlePaddle supports a variety of flexible and efficient recurrent neural networks. For details, please refer to：
 
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   :maxdepth: 1
 
-  rnn/index_cn.rst
+  rnn/index_en.rst
 
 How to use the built-in timing tool, nvprof, or nvvp to run performance analysis and tuning, please refer to：
 
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   :maxdepth: 1
 
-  optimization/gpu_profiling_cn.rst
+  optimization/gpu_profiling_en.rst