fix sample code

python/paddle/fluid/layers/nn.py

@@ -6102,8 +6102,10 @@ def reshape(x, shape, actual_shape=None, act=None, inplace=False, name=None):
     Examples:
         .. code-block:: python
             
+            import paddle
             import paddle.fluid as fluid
             paddle.enable_static()
+            
             # example 1:
             # attr shape is a list which doesn't contain Tensors.
             data_1 = fluid.data(