add brief explanation of DDim usage

paddle/majel/README.md

@@ -48,9 +48,7 @@ int main()
 }
 ```
 
-In Majel, `DDimVar` is derived from `Dim`, `DArrayVar` is from 
-`Array`.
-
+In Majel, `DDimVar` is derived from `Dim`, `DArrayVar` is from `Array`.
 
 ```c++
 template<int i>
@@ -108,7 +106,6 @@ Because `variant` may be thought of as "multi-type, single value", we can utiliz
  arr[make_ddim({0, 1})] = 1.0；
  ```
 
-
 ## implement Tensor in Paddle
 
 Before writing code, please make sure you already look through Majel Source Code and grabbed the design philosophy of `DArray` in Majel.