minor fixes in readMe and ios function

README.md

@@ -373,12 +373,14 @@ class App extends Component {
           my_custom_data_2: 'my_custom_field_value_2'
         });
         
-        FCM.deleteInstanceId().then(()={
+        FCM.deleteInstanceId()
+            .then( () => {
               //Deleted instance id successfully
               //This will reset Instance ID and revokes all tokens.
-        }).catch(error =>{
-          //Error while deleting instance id 
             })
+            .catch(error => {
+              //Error while deleting instance id 
+            });
     }
 }
 ```

ios/RNFIRMessaging.m

@@ -236,9 +236,9 @@ RCT_EXPORT_METHOD(deleteInstanceId:(RCTPromiseResolveBlock)resolve rejecter:(RCT
   [[FIRInstanceID instanceID]deleteIDWithHandler:^(NSError * _Nullable error) {
     
     if (error != nil) {
-      reject([NSString stringWithFormat:@"%ld",error.code],error.localizedDescription,error);
+      reject([NSString stringWithFormat:@"%ld",error.code],error.localizedDescription,nil);
     } else {
-      resolve(NULL);
+      resolve(nil);
     }
   }];
 }