docs: fix setWindowOpenHandler invocation in window.open example

[kdau]

Description of Change

The "Native Window example" in docs/api/window-open.md calls webContents.setWindowOpenHandler incorrectly. This PR fixes the call, in line with previous fixes to other calls in this document.

Checklist

• PR description included and stakeholders cc'd
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed invocation of webContents.setWindowOpenHandler in window.open example.

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Release Notes Persisted

Fixed invocation of webContents.setWindowOpenHandler in window.open example.

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I have automatically backported this PR to "12-x-y", please check out #29725

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I have automatically backported this PR to "13-x-y", please check out #29726

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I have automatically backported this PR to "14-x-y", please check out #29727