Update index_cn.rst

doc/fluid/api_guides/index_cn.rst

-API Guides
+API Guide
 =========
 
 ..  toctree::
@@ -6,4 +6,4 @@ API Guides
 
   :maxdepth: 1
   
-  high_low_level_api.md
\ No newline at end of file
+  high_low_level_api.md