[DOC][PYTHON] Fix api comment of paddle.fluid.clip.GradientClipByValue (#17530)

* BugFix: fix api comment of paddle.fluid.clip.GradientClipByValue
* test=develop, test=document_preview

python/paddle/fluid/clip.py

@@ -134,12 +134,13 @@ class GradientClipByValue(BaseGradientClipAttr):
     Examples:
         .. code-block:: python
 
+            import paddle.fluid as fluid
             w_param_attrs = fluid.ParamAttr(name=None,
               initializer=fluid.initializer.UniformInitializer(low=-1.0, high=1.0, seed=0),
               learning_rate=1.0,
               regularizer=fluid.regularizer.L1Decay(1.0),
               trainable=True,
-              clip=fluid.clip.GradientClipByValue(-1.0, 1.0))
+              clip=fluid.clip.GradientClipByValue(1.0, -1.0))
             y_predict = fluid.layers.fc(input=x, size=1, param_attr=w_param_attrs)
     """