chore: Fix self-referential dependency in MongoDB Atlas private endpoint example

[sreeteja06]

Proposed changes

Fixed an incorrect self-referential dependency in the MongoDB Atlas Private Endpoint CDK example.

The original code added a dependency of a resource on itself, which is logically incorrect and could potentially cause deployment issues. The fix replaces the self-dependency with a dependency on the AWS VPC endpoint to ensure correct resource creation order.

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as
  expected)
• This change requires a documentation update

Required Checklist:

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• I have tested the CDK constructor in a CFN stack. See TESTING.md
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

This is a minor fix to improve the correctness of the example code's dependency management.

[marcosuma]

Thank you for your PR @sreeteja06!