fix code snippet (#8146)

Author: swaminator

src/pages/[platform]/build-a-backend/auth/concepts/external-identity-providers/index.mdx

@@ -225,7 +225,7 @@ import { defineAuth } from '@aws-amplify/backend';
 
 export const auth = defineAuth({
   loginWith: {
-    externalAuthProviders: {
+    externalProviders: {
       loginWithAmazon: {
         clientId: secret('LOGINWITHAMAZON_CLIENT_ID'),
         clientSecret: secret('LOGINWITHAMAZON_CLIENT_SECRET'),