Fix links in glm_image.md

[oglok]

Purpose

Fix 3 broken relative links in docs/user_guide/examples/online_serving/glm_image.md that cause the ReadTheDocs build to fail in strict mode.

The links use ../../../../serving/ (4 levels up) when they should use ../../../serving/ (3 levels up). The file is at docs/user_guide/examples/online_serving/glm_image.md, so reaching docs/user_guide/serving/ requires going up 3 directories, not 4.

Introduced in PR #2051 ([Doc] Improve diffusion generation parameter docs for online serving, commit d96d63cb).

WARNING - Doc file 'user_guide/examples/online_serving/glm_image.md' contains a link
  '../../../../serving/diffusion_chat_api.md', but the target '../serving/diffusion_chat_api.md'
  is not found among documentation files. Did you mean '../../../serving/diffusion_chat_api.md'?

This currently blocks the ReadTheDocs CI for all open PRs.

Test Plan

python -m mkdocs build --clean --site-dir /tmp/mkdocs-output --config-file mkdocs.yml --strict

No additional test scripts needed -- this is a documentation-only fix.

Test Result

Before: Aborted with 3 warnings in strict mode!

After: 0 warnings from glm_image.md. Build completes successfully.

---

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• The test plan. Please provide the test scripts & test commands.
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• (Optional) The necessary documentation update.
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