Update reference-react-component.md (#609)

`just before` sounds better than `immediately before`.

content/docs/reference-react-component.md

@@ -186,7 +186,7 @@ If you "fork" props by using them for state, you might also want to implement [`
 componentWillMount()
 ```
 
-`componentWillMount()` is invoked immediately before mounting occurs. It is called before `render()`, therefore calling `setState()` synchronously in this method will not trigger an extra rendering. Generally, we recommend using the `constructor()` instead.
+`componentWillMount()` is invoked just before mounting occurs. It is called before `render()`, therefore calling `setState()` synchronously in this method will not trigger an extra rendering. Generally, we recommend using the `constructor()` instead.
 
 Avoid introducing any side-effects or subscriptions in this method. For those use cases, use `componentDidMount()` instead.