fix(ts types): Fixing $once type declaration on vue.d.ts file. $once should accept an event or array

Fixing $once type declaration on vue.d.ts file. $once should accept an event or array of events as
first paramete

fix vuejs#8983

Author: Hugo Azevedo

types/vue.d.ts

@@ -53,7 +53,7 @@ export interface Vue {
     options?: WatchOptions
   ): (() => void);
   $on(event: string | string[], callback: Function): this;
-  $once(event: string, callback: Function): this;
+  $once(event: string | string[], callback: Function): this;
   $off(event?: string | string[], callback?: Function): this;
   $emit(event: string, ...args: any[]): this;
   $nextTick(callback: (this: this) => void): void;