docs: complete the example in use InjectionToken section (#57839)

PR Close #57839

adev/src/content/guide/di/dependency-injection-providers.md

@@ -161,6 +161,10 @@ The following example defines a token, `APP_CONFIG`. of the type `InjectionToken
 <docs-code header="src/app/app.config.ts" language="typescript" highlight="[3]">
 import { InjectionToken } from '@angular/core';
 
+export interface AppConfig {
+  title: string;
+}
+
 export const APP_CONFIG = new InjectionToken<AppConfig>('app.config description');
 </docs-code>
 
@@ -169,6 +173,10 @@ The optional type parameter, `<AppConfig>`, and the token description, `app.conf
 Next, register the dependency provider in the component using the `InjectionToken` object of `APP_CONFIG`:
 
 <docs-code header="src/app/app.component.ts" language="typescript">
+const MY_APP_CONFIG_VARIABLE: AppConfig = {
+  title: 'Hello',
+};
+
 providers: [{ provide: APP_CONFIG, useValue: MY_APP_CONFIG_VARIABLE }]
 </docs-code>