Merge pull request #382 from tink2123/fix_reader

fix reader.md

doc/fluid/user_guides/howto/prepare_data/reader.md

-```eval_rst
-.. _user_guide_reader:
-```
-
 # Python Reader
 
 During the training and testing phases, PaddlePaddle programs need to read data. To help the users write code that performs reading input data, we define the following: