docs: remove extra anonymous function in preview docs

Co-authored-by: Roland Groza <[email protected]>

examples/previews/README.md

@@ -60,7 +60,7 @@ function Previews(props) {
     </div>
   ));
 
-  useEffect(() => () => {
+  useEffect(() => {
     // Make sure to revoke the data uris to avoid memory leaks
     files.forEach(file => URL.revokeObjectURL(file.preview));
   }, [files]);