Add example for how to use style properly

Test Plan: jest

src/browser/ui/ReactDOMComponent.js

@@ -69,7 +69,8 @@ function assertValidProps(props) {
   invariant(
     props.style == null || typeof props.style === 'object',
     'The `style` prop expects a mapping from style properties to values, ' +
-    'not a string.'
+    'not a string. For example, style={{marginRight: spacing + \'em\'}} when ' +
+    'using JSX.'
   );
 }