[package-json] fix wrong return type

I made a mistake in my original PR
https://github.com/DefinitelyTyped/DefinitelyTyped/pull/21578

The function should return a promise.
- https://www.npmjs.com/package/package-json
- https://github.com/sindresorhus/package-json/blob/master/index.js#L30

types/package-json/index.d.ts

@@ -9,6 +9,6 @@ interface PackageJsonOptions {
     allVersions?: boolean;
 }
 
-declare function packageJson(name: string, options?: PackageJsonOptions): {};
+declare function packageJson(name: string, options?: PackageJsonOptions): Promise<{}>;
 
 export = packageJson;

types/package-json/package-json-tests.ts

@@ -1,7 +1,10 @@
 import packageJson = require('package-json');
 
-const pkg = packageJson('package-json', {
+let pkg: {};
+packageJson('package-json', {
     version: '1.2.3',
     fullMetadata: true,
     allVersions: true,
+}).then(x => {
+    pkg = x;
 });