Update index_en.rst

fix space and toctree format

Author: shanyi15

doc/v2/getstarted/index_en.rst

@@ -10,9 +10,10 @@ If you want to quickly know how to use PaddlePaddle, please refer to the followi
 
 
 While using PaddlePaddle to build applications, please understand some basic concepts.
+
 Here is an example of linear regression. It introduces workflow of PaddlePaddle, including data format, model configuration and training, etc.
 
-  ..  toctree::
+..  toctree::
   :maxdepth: 1
 
   concepts/use_concepts_en.rst