Add missing "default" to the type definition

Author: nwtgck

types/index.d.ts

@@ -1,4 +1,4 @@
 import { IAsyncComputedValue } from 'vue-async-computed';
 import { VueDecorator } from 'vue-class-component';
 
-export function AsyncComputed<T>(computedOptions?: IAsyncComputedValue<T>): VueDecorator;
+export default function AsyncComputed<T>(computedOptions?: IAsyncComputedValue<T>): VueDecorator;