fix double buffer example (#18169)

test=develop
test=document_preview

paddle/fluid/API.spec

@@ -246,7 +246,7 @@ paddle.fluid.layers.open_files (ArgSpec(args=['filenames', 'shapes', 'lod_levels
 paddle.fluid.layers.read_file (ArgSpec(args=['reader'], varargs=None, keywords=None, defaults=None), ('document', '32181f6037e387fb6e68a5beaafe33b6'))
 paddle.fluid.layers.shuffle (ArgSpec(args=['reader', 'buffer_size'], varargs=None, keywords=None, defaults=None), ('document', 'f29d7d159e114f73fc988d9a86805841'))
 paddle.fluid.layers.batch (ArgSpec(args=['reader', 'batch_size'], varargs=None, keywords=None, defaults=None), ('document', 'fcb24383c6eef2ca040ee824c26e22fd'))
-paddle.fluid.layers.double_buffer (ArgSpec(args=['reader', 'place', 'name'], varargs=None, keywords=None, defaults=(None, None)), ('document', 'c13b8a8521bea5f8123b925ae2a5d5db'))
+paddle.fluid.layers.double_buffer (ArgSpec(args=['reader', 'place', 'name'], varargs=None, keywords=None, defaults=(None, None)), ('document', '7241dd1c142f4c65c8d7f66948140aa7'))
 paddle.fluid.layers.random_data_generator (ArgSpec(args=['low', 'high', 'shapes', 'lod_levels', 'for_parallel'], varargs=None, keywords=None, defaults=(True,)), ('document', '9b7f0f86ec24bbc97643cadcb6499cff'))
 paddle.fluid.layers.py_reader (ArgSpec(args=['capacity', 'shapes', 'dtypes', 'lod_levels', 'name', 'use_double_buffer'], varargs=None, keywords=None, defaults=(None, None, True)), ('document', 'd78a1c7344955c5caed8dc13adb7beb6'))
 paddle.fluid.layers.create_py_reader_by_data (ArgSpec(args=['capacity', 'feed_list', 'name', 'use_double_buffer'], varargs=None, keywords=None, defaults=(None, True)), ('document', '2edf37d57862b24a7a26aa19a3573f73'))

python/paddle/fluid/layers/io.py

@@ -1092,6 +1092,7 @@ def double_buffer(reader, place=None, name=None):
         >>> import paddle.fluid as fluid
         >>> reader = fluid.layers.open_files(filenames=['mnist.recordio'],
         >>>                                  shapes=[[-1, 784], [-1, 1]],
+        >>>                                  lod_levels=[0, 0],
         >>>                                  dtypes=['float32', 'int64'])
         >>> reader = fluid.layers.double_buffer(reader)
         >>> img, label = fluid.layers.read_file(reader)