[guide][react] Add missing parenthesis in «Refs» recommendation

As is, actual «good» usage fires the arrow-parens rule.
Parenthesis are needed to follow the current style guide.

Author: soulchainer

react/README.md

@@ -378,7 +378,7 @@
 
     // good
     <Foo
-      ref={ref => { this.myRef = ref; }}
+      ref={(ref) => { this.myRef = ref; }}
     />
     ```