Use correct `[[targets.distros]]` schema in RFC 0128

[edmorley]

Updates RFC 0128 to use the correct [[targets.distros]] schema as defined already in the spec: https://github.com/buildpacks/spec/blob/main/buildpack.md#buildpacktoml-toml

I've not added an ## Amended section, since:

1. The RFC has only just merged (this is effectively a review comment that missed the merge by a day)
2. The parts of the RFC being fixed are existing concepts already in the spec, rather than design decisions relating to the purpose of the RFC itself.

Fixes https://github.com/buildpacks/rfcs/pull/295#discussion_r1559614121.

[buildpack-bot]

Maintainers,

As you review this RFC please queue up issues to be created using the following commands:

/queue-issue <repo> "<title>" [labels]...
/unqueue-issue <uid>

Issues

(none)

[edmorley]

@jjbustamante Could I have review? :-)

[jjbustamante]

Just to be consistent with the process, I think we should add the Amend section at the bottom

<!--
## Amended
### Meta
[meta-1]: #meta-1
- Name: (fill in the amendment name: Variable Rename)
- Start Date: (fill in today's date: YYYY-MM-DD)
- Author(s): (Github usernames)
- Amendment Pull Request: (leave blank)

### Summary

A brief description of the changes.

### Motivation

Why was this amendment necessary?
--->

[edmorley]

Just to be consistent with the process, I think we should add the Amend section at the bottom

I explained why I didn't add this section in the PR description. I still think we shouldn't add one.

[edmorley]

Ultimately this is a typo fix to something unrelated to the RFC, not a change in the actual intent of the RFC.

[edmorley]

Please can someone merge this?

[edmorley]

Thank you :-)