fix README.md floating-ui example code

It was using the user defined `opts` without the default `popperOptions` that were defined just above (that are overridden by the user options).

README.md

@@ -118,7 +118,7 @@ function popperFactory(ref, content, opts) {
  }
 
  function update() {
- computePosition(ref, content, opts).then(({x, y}) => {
+ computePosition(ref, content, popperOptions).then(({x, y}) => {
  Object.assign(content.style, {
  left: `${x}px`,
  top: `${y}px`,