Minor typo in README.md

## Documentation


In the [introduction session of validate rich menu object api](https://github.com/line/line-bot-sdk-python#validate_rich_menu_objectself-rich_menu-timeoutnone), The last line of example should be
```
line_bot_api.validate_rich_menu_object(rich_menu=rich_menu_to_validate)
```