fix(docs): add a necessary await operator to the example code for initializeAppCheck() (#8569)

Co-authored-by: Aike <[email protected]>

Author: aike19115

docs/app-check/usage/index.md

@@ -201,7 +201,7 @@ Once you have the custom provider configured, install it in app-check using the
 import { getApp } from '@react-native-firebase/app';
 import { initializeAppCheck } from '@react-native-firebase/app-check';
 
-const appCheck = initializeAppCheck(getApp(), {
+const appCheck = await initializeAppCheck(getApp(), {
   provider: rnfbProvider,
   isTokenAutoRefreshEnabled: true,
 });