docs: fix wrong code example for preload property

[Paratron]

Description of Change

The docs for the process model mention that preload should be a property of webPreferences, but the code example showed something else.

notes: none

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[erickzhao]

Thank you!

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Congrats on merging your first pull request! 🎉🎉🎉

[release-clerk]

No Release Notes

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I was unable to backport this PR to "14-x-y" cleanly;
you will need to perform this backport manually.

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I have automatically backported this PR to "13-x-y", please check out #30872

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I was unable to backport this PR to "15-x-y" cleanly;
you will need to perform this backport manually.

[erickzhao]

Removing bp label for 14-x-y because #30800