Fix path argument in iOS excludeFromBackupKey (#473)

ios.js

  * @param  {string} url URL of the resource, only file URL is supported
  * @return {Promise}
  */
-function excludeFromBackupKey(url:string) {
+function excludeFromBackupKey(path:string) {
   return RNFetchBlob.excludeFromBackupKey('file://' + path);
 }