fix docs bugs

[Tsaiyue]

PR types

Bug fixes

PR changes

Docs

Description

涉及到的API有

• paddle.nn.functional.softmax_with_cross_entropy
• paddle.nn.functional.base_softmax_with_cross_entropy

bug描述：API英文文档的返回结果（Returns）描述格式有误，与对应中文文档不一致；
注: 该PR源于启航计划集训营热身打卡任务。

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