Update README.md

Removing braces and parentheses so that the example doesn't break. :)

README.md

         font-size: 16px;
       }
     `}
-    onSizeUpdated={({size => console.log(size.height)})},
+    onSizeUpdated={size => console.log(size.height)}
     files={[{
         href: 'cssfileaddress',
         type: 'text/css',