update google auth provider example to use entity ref for the token subject

plugin-auth-backend broke started requiring a full entity ref on version [0.12.0](https://github.com/backstage/backstage/blob/master/plugins/auth-backend/CHANGELOG.md#0120)

see:
https://github.com/backstage/backstage/releases/tag/v0.70.0
https://github.com/backstage/backstage/blob/master/plugins/auth-backend/CHANGELOG.md#0120
https://github.com/backstage/backstage/commit/0c8ba31d72670d9ff5eda3ca726e061fff088115
Signed-off-by: João Fernandes <joaofnds@joaofnds.com>

docs/auth/identity-resolver.md

@@ -51,11 +51,12 @@ export default async function createPlugin(
             // Let's use the username in the email ID as the user's default
             // unique identifier inside Backstage.
             const [id] = email.split('@');
-            ent.push(stringifyEntityRef({
+            const userEntityRef = stringifyEntityRef({
               kind: 'User',
               namespace: DEFAULT_NAMESPACE,
               name: id,
-            }));
+            });
+            ent.push(userEntityRef);
 
             // Let's call the internal LDAP provider to get a list of groups
             // that the user belongs to, and add those to the list as well
@@ -68,7 +69,7 @@ export default async function createPlugin(
 
             // Issue the token containing the entity claims
             const token = await ctx.tokenIssuer.issueToken({
-              claims: { sub: id, ent },
+              claims: { sub: userEntityRef, ent },
             });
             return { id, token };
           },
@@ -130,6 +131,8 @@ can do this using the `CatalogIdentityClient` provided as context to Sign-In
 resolvers:
 
 ```ts
+import { DEFAULT_NAMESPACE, stringifyEntityRef } from '@backstage/catalog-model';
+
 export default async function createPlugin(
   env: PluginEnvironment,
 ): Promise<Router> {
@@ -140,6 +143,11 @@ export default async function createPlugin(
         signIn: {
           resolver: async ({ profile: { email } }, ctx) => {
             const [id] = email?.split('@') ?? '';
+            const userEntityRef = stringifyEntityRef({
+              kind: 'User',
+              namespace: DEFAULT_NAMESPACE,
+              name: id,
+            });
             // Fetch from an external system that returns entity claims like:
             // ['user:default/breanna.davison', ...]
             const ent = await externalSystemClient.getUsernames(email);
@@ -150,7 +158,7 @@ export default async function createPlugin(
               logger: ctx.logger,
             });
             const token = await ctx.tokenIssuer.issueToken({
-              claims: { sub: id, ent: fullEnt },
+              claims: { sub: userEntityRef, ent: fullEnt },
             });
             return { id, token };
           },