[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.

開啟連結的 Add React to a website 文件頁面，找到「Add React in One Minute」區段。閱讀步驟 3：Create a React Component，並釐清所參照程式碼片段中的 e 代表什麼。當程式碼片段或周圍的文件能讓新讀者理解 e 時，即表示完成。