Fix typo

README.md

 ```ts
 interface NotificationSettings {
   // properties only availables on iOS
-  // unavaible settings will not be included in the response object
+  // unavailable settings will not be included in the response object
   alert?: boolean;
   badge?: boolean;
   sound?: boolean;
 
 interface NotificationSettings {
   // properties only availables on iOS
-  // unavaible settings will not be included in the response object
+  // unavailable settings will not be included in the response object
   alert?: boolean;
   badge?: boolean;
   sound?: boolean;