Fix the error of op `ones_like` document，change the output variable

test=document_preview
test=develop
Fix the error of op `ones_like` document, change the output variable from x to out.

paddle/fluid/API.spec

@@ -308,7 +308,7 @@ paddle.fluid.layers.isfinite (ArgSpec(args=['x'], varargs=None, keywords=None, d
 paddle.fluid.layers.range (ArgSpec(args=['start', 'end', 'step', 'dtype'], varargs=None, keywords=None, defaults=None), ('document', 'a45b42f21bc5a4e84b60981a3d629ab3'))
 paddle.fluid.layers.linspace (ArgSpec(args=['start', 'stop', 'num', 'dtype'], varargs=None, keywords=None, defaults=None), ('document', '3663d1148946eed4c1c34c81be586b9e'))
 paddle.fluid.layers.zeros_like (ArgSpec(args=['x', 'out'], varargs=None, keywords=None, defaults=(None,)), ('document', 'd88a23bcdc443719b3953593f7cef14a'))
-paddle.fluid.layers.ones_like (ArgSpec(args=['x', 'out'], varargs=None, keywords=None, defaults=(None,)), ('document', '642afd126553337d6796600e886a6525'))
+paddle.fluid.layers.ones_like (ArgSpec(args=['x', 'out'], varargs=None, keywords=None, defaults=(None,)), ('document', 'd18d42059c6b189cbd3fab2fcb206c15'))
 paddle.fluid.layers.diag (ArgSpec(args=['diagonal'], varargs=None, keywords=None, defaults=None), ('document', '88a15e15f0098d549f07a01eaebf9ce3'))
 paddle.fluid.layers.eye (ArgSpec(args=['num_rows', 'num_columns', 'batch_shape', 'dtype'], varargs=None, keywords=None, defaults=(None, None, 'float32')), ('document', '25389d1e239a5d1cda66298f908ec549'))
 paddle.fluid.layers.While ('paddle.fluid.layers.control_flow.While', ('document', '50110155608a00f43d3d3fd1be41dcb4'))

python/paddle/fluid/layers/tensor.py

@@ -1074,7 +1074,7 @@ def ones_like(x, out=None):
         out(Variable): The output tensor.
 
     Returns:
-        x(Variable): The tensor variable storing the output.
+        out(Variable): The tensor variable storing the output.
 
     Examples:
         .. code-block:: python