Fix title consistency from "Transformer Reinforcement Learning" to "Transformers Reinforcement Learning"

[qgallouedec]

What does this PR do?

Fixes # (issue)

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---

Note

Low Risk
Docs-only wording change to the project title in README.md and the docs landing page; no code, behavior, or APIs are affected.

Overview
Updates the displayed TRL project title from "Transformer Reinforcement Learning" to "Transformers Reinforcement Learning" for consistency across README.md and docs/source/index.md.

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[albertvillanova]

I would prefer others to give their opinion.

I always thought the right name was "Transformers Reinforcement Learning":

• short of "Transformers with Reinforcement Learning"

[qgallouedec]

democracy

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[qgallouedec]

For the banner: https://huggingface.co/datasets/trl-lib/documentation-images/discussions/6

[albertvillanova]

Thanks.