fix(amplify): addBranch fails synth with "cannot find entry file..." (#28772)

#28658 missed changing one of the paths (i.e it didn't remove the extra '..') and I was still getting the "cannot find entry file" error. I can confirm that my CDK stack works as expected after making the change locally.

This is my first CDK PR. I did the following:

```
yarn install
npx lerna run build --scope=@aws-cdk/aws-amplify-alpha
cd packages/@aws-cdk/aws-amplify-alpha
yarn build
yarn test
yarn integ-runner --update-on-failed
npx lerna run build --scope=aws-cdk-lib
```

Closes #28764

----

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

Author: brettstack

packages/@aws-cdk/aws-amplify-alpha/lib/branch.ts

@@ -254,7 +254,7 @@ class AmplifyAssetDeploymentProvider extends NestedStack {
       this,
       'amplify-asset-deployment-is-complete',
       {
-        entry: path.join(__dirname, '..', '..', 'custom-resource-handlers', 'dist', 'aws-amplify-alpha', 'asset-deployment-handler', 'index.js'),
+        entry: path.join(__dirname, '..', 'custom-resource-handlers', 'dist', 'aws-amplify-alpha', 'asset-deployment-handler', 'index.js'),
         handler: 'isComplete',
         initialPolicy: [
           new iam.PolicyStatement({

packages/@aws-cdk/aws-amplify-alpha/test/integ.app-asset-deployment.js.snapshot/asset.9307ab790c544d7c4d94504a1c950222bbfc119c2c12b44440ad46939e0b6364/index.js

@@ -90,7 +90,7 @@
        {
         "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
        },
-       "/2f9d2632b48a4f92f6691c3e3cfcbeb47798c945a2d03b93514de1a3d3ec72ed.json"
+       "/d43fb7d2905864414e4778c7de7f1a874634c94930fb3c2e0ce4aef8d44fcbbd.json"
       ]
      ]
     }

packages/@aws-cdk/aws-amplify-alpha/test/integ.app-asset-deployment.js.snapshot/cdk-amplify-app-asset-deployment.assets.json

@@ -144,7 +144,7 @@
      "S3Bucket": {
       "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
      },
-     "S3Key": "9307ab790c544d7c4d94504a1c950222bbfc119c2c12b44440ad46939e0b6364.zip"
+     "S3Key": "85f7bb0ac0b4da3571afd49c891d631f4f07c0ef710a02046d3a9abd8d9d4604.zip"
     },
     "Environment": {
      "Variables": {