docs: wrong structured output example in sdk docs

[ppodds]

Description

The example included in #8161 is wrong. The structured output result is stored in structured not structured_output.

Plugins

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OpenCode version

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Steps to reproduce

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Operating System

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Terminal

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[github-actions]

To stay organized issues are automatically closed after 60 days of no activity. If the issue is still relevant please open a new one.

[corsin-ragettli]

I don't even have the possibility to use structured output in the correct version of opencode (1.17.11), how did you get to use it?