Update io.md

没有image这个迭代器，我把配置文件改成img好像就可以了.

doc/io.md

@@ -14,7 +14,7 @@ options 2 =
 ...
 iter = end
 ```
-* The basic iterator type is **mnist** , **image** , **imgrec**, **csv**
+* The basic iterator type is **mnist** , **img** , **imgrec**, **csv**
 * To use thread buffer, declare in this form
 ```bash
 iter = iterator_type