chore(cognito): fix test case name for User Pool Domain (#31365)

While working on #31351, I discovered.
The test case name for `User Pool Domain` was incorrectly set as `User Pool Client`. 

It's likely that when the code was reused from `user-pool-client.test.ts`, the test case name wasn't updated.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mazyu36

packages/aws-cdk-lib/aws-cognito/test/user-pool-domain.test.ts

@@ -3,7 +3,7 @@ import { Certificate } from '../../aws-certificatemanager';
 import { CfnParameter, Stack } from '../../core';
 import { UserPool, UserPoolDomain } from '../lib';
 
-describe('User Pool Client', () => {
+describe('User Pool Domain', () => {
   test('custom domain name', () => {
     // GIVEN
     const stack = new Stack();