[Documentation] Add react in one minute

Steps to reproduce:

• Go to Add React to a website section [https://reactjs.org/docs/add-react-to-a-website.html#add-react-in-one-minute](docs url)

• In Add React in One Minute section search for the step Step 3: Create a React Component

• Code snippet reference attached doesn't mention what e means making the code confusing.

打开链接的 Add React to a website 文档页面，找到“Add React in One Minute”部分。阅读步骤 3：Create a React Component，并说明所引用代码片段中的 e 表示什么。当代码片段或周围的文档能够让新读者理解 e 时，即视为完成。