[DOC] Update README.rst

Author: xuyxu

README.rst

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-Deep Forest
-===========
+Deep Forest (DF) 21
+===================
 
 |github|_ |codecov|_ |python|_ |pypi|_
 
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 .. |pypi| image:: https://img.shields.io/pypi/v/deep-forest?color=blue
 .. _pypi: https://pypi.org/project/deep-forest/
 
-Deep Forest (DF) 21
-===================
-
 **DF21** is an implementation of Deep Forest 2021.2.1. It is designed to have the following advantages:
 
 - **Powerful**: Better accuracy than existing tree-based ensemble methods.