MonthPicker component correction

[xkim1994]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

开始以为直接引入MonthPicker组件就可以使用没想到在DatePicker的下一级，希望即使纠正更新文档

What does the proposed API look like?

Vue.component(MonthPicker.name, MonthPicker)

[tangjinzhou]

推荐直接使用 Vue.use(DatePicker) 会自动为你注册子组件

[github-actions]

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