Modify control_flow.py doc

Author: kexinzhao

doc/fluid/design/concepts/lod_tensor.md

@@ -198,7 +198,7 @@ Hence, we encapsulate this implementation detail in C++ and expose the original
 Specifically, user can use the following code to set or get the LoD info of a LoDTensor in Python:
 ```Python
 # lod using length representation
-lod = [[3, 1, 2], [2, 2, 1, 3, 1, 2]]
+lod = [[1, 3, 2]]
 # Create a LoDTensor that has the above lod info.
 # This lod will be converted to an offset representation in the C++ implementation under the hood.
 tensor = fluid.LoDTensor(lod)

python/paddle/fluid/layers/control_flow.py

@@ -746,8 +746,8 @@ def lod_rank_table(x, level=0):
         .. code-block:: text
 
             x is a LoDTensor:
-                x.lod = [[0,                2, 3],
-                         [0,             5, 6, 7]]
+                x.lod = [[2,                1],
+                         [5,             1, 1]]
                 x.data = [a, b, c, d, e, f, g]
 
             1. set level to 0: