Backport: docs: change the highlighted lines for code block

[github-actions]

This is an automated backport of #14855 to the release-v6.0 branch. FYI @aayush-kapoor
This backport has conflicts that need to be resolved manually.

git cherry-pick output

Auto-merging content/providers/01-ai-sdk-providers/16-google-vertex.mdx
CONFLICT (content): Merge conflict in content/providers/01-ai-sdk-providers/16-google-vertex.mdx
error: could not apply cf517e12e... docs: change the highlighted lines for code block (#14855)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"