docs: fix optional argument representation (#4732)

Author: kbrandwijk

content/docs/reference-react-component.md

@@ -507,7 +507,7 @@ There are just two of them: `setState()` and `forceUpdate()`.
 ### `setState()` {#setstate}
 
 ```javascript
-setState(updater, [callback])
+setState(updater[, callback])
 ```
 
 `setState()` enqueues changes to the component state and tells React that this component and its children need to be re-rendered with the updated state. This is the primary method you use to update the user interface in response to event handlers and server responses.