[Doc] Fix typos in speculative decoding docs

[lkm2835]

Purpose

This PR fixes typos in the speculative decoding documentation.

Test Plan

Test Result

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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.

[claude]

Claude Code Review

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[mergify]

Documentation preview: https://vllm--51500.org.readthedocs.build/en/51500/

[lkm2835]

Hi @hmellor!
Not sure if this falls under your area, but could you review this PR?
It only contains a few minor typo fixes in the documentation. Thanks!

[github-actions]

✅ @lkm2835, CI is now available for this PR.

• /ci run starts a CI build.
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[hmellor]

(I updated the branch because I just merged a PR that fixes the docs build on main)

[hmellor]

/ci run

[github-actions]

✅ Triggered Buildkite CI #83311 for commit 8fa4ad9adc8d.