Fix createBranchUniversalObject's TypeScript definition

createBranchUniversalObject is an async function, so it's TypeScript definition should be a function that returns `Promise<BranchUniversalObject>` instead of `BranchUniversalObject`.

Author: v-fernandez

src/index.d.ts

@@ -309,7 +309,7 @@ interface Branch {
   createBranchUniversalObject: (
     identifier: string,
     options: BranchUniversalObjectOptions
-  ) => BranchUniversalObject;
+  ) => Promise<BranchUniversalObject>;
   handleATTAuthorizationStatus: (
     ATTAuthorizationStatus:ATTAuthorizationStatus
   ) => void