[Dy2Stat] Refine code example for 2.0 (#28440)

python/paddle/fluid/dygraph/jit.py

@@ -176,24 +176,20 @@ def declarative(function=None, input_spec=None):
     Examples:
         .. code-block:: python
 
-          import paddle.fluid as fluid
-          import numpy as np
-          from paddle.fluid.dygraph.jit import declarative
-
-          fluid.enable_dygraph()
+            import paddle
+            from paddle.jit import to_static
 
-          @declarative
+            @to_static
             def func(x):
-              x = fluid.dygraph.to_variable(x)
-              if fluid.layers.mean(x) < 0:
+                if paddle.mean(x) < 0:
                     x_v = x - 1
                 else:
                     x_v = x + 1
                 return x_v
 
-          x = np.ones([1, 2])
+            x = paddle.ones([1, 2], dtype='float32')
             x_v = func(x)
-          print(x_v.numpy()) # [[2. 2.]]
+            print(x_v) # [[2. 2.]]
 
     """