docs: Correct reference module implementation names to be unique in v20 upgrade guide

[bryantbiggs]

Description

• Correct reference module implementation names to be unique in v20 upgrade guide

Motivation and Context

• https://github.com/terraform-aws-modules/terraform-aws-eks/issues/3136#issuecomment-2398327975

Breaking Changes

• No

How Has This Been Tested?

• [ ] I have updated at least one of the examples/* to demonstrate and validate my change(s)
• [ ] I have tested and validated these changes using one or more of the provided examples/* projects
• [ ] I have executed pre-commit run -a on my pull request

[antonbabenko]

This PR is included in version 20.25.0 :tada:

[github-actions[bot]]

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