docs: Add backquote to default value

components/button/index.en-US.md

@@ -21,7 +21,7 @@ htmlType | to set the original `type` of `button`, see: [MDN](https://developer.
 icon | set the icon of button, see: Icon component | string | -
 shape | can be set to `circle` or omitted | string | -
 size | can be set to `small` `large` or omitted | string | `default`
-loading | to set the loading status of button | boolean | false
+loading | to set the loading status of button | boolean | `false`
 onClick | set the handler to handle `click` event | function | -
 
 `<Button>Hello world!</Button>` will be rendered into `<button>Hello world!</button>`, and all the properties which are not listed above will be transferred to the `<button>` tag.

components/button/index.zh-CN.md

@@ -24,7 +24,7 @@ htmlType | 设置 `button` 原生的 `type` 值，可选值请参考 [HTML 标
 icon | 设置按钮的图标类型 | string | -
 shape | 设置按钮形状，可选值为 `circle` 或者不设 | string | -
 size | 设置按钮大小，可选值为 `small` `large` 或者不设 | string | `default`
-loading | 设置按钮载入状态 | boolean | false
+loading | 设置按钮载入状态 | boolean | `false`
 onClick | `click` 事件的 handler | function | -
 
 `<Button>Hello world!</Button>` 最终会被渲染为 `<button>Hello world!</button>`，并且除了上表中的属性，其它属性都会直接传到 `<button></button>`。