fixing vllm adapter install.

[NicoGrande]

Description

Fixing path to MaxText vLLM adapter plugin.

Tests

uv pip install -e .[tpu-post-train] --resolution=lowest

install_maxtext_tpu_post_train_extra_deps

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[richjames0]

lgtm

[bvandermoon]

LGTM, thank you @NicoGrande