[Doc] Fix typo in serving docs

[the-codeboy]

Purpose

Fixed two typos in the serving docs about the open ai compatible api

Test Plan

Didn't change any content, just two typos:
how are roles, messages, and other chat-specific tokens are encoded in the input -> how roles, messages, and other chat-specific tokens are encoded in the input
For those model -> For those models

Test Result

everything still works since i only fixed typos

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

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[mergify]

Documentation preview: https://vllm--28474.org.readthedocs.build/en/28474/

[gemini-code-assist]

Code Review

This pull request fixes two minor typos in the openai_compatible_server.md documentation file. The changes are correct and improve the clarity of the text. The PR is well-scoped and ready to be merged.

[tjtanaa]

@the-codeboy LGTM. Thank you for proof reading.