Fix list update typing on list to enable partial updates

[vicrac]

I added Partial type around type parameter in update operation, to enable partial updates, as claimed in docs

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[vicrac]

@googlebot I signed it!

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CLAs look good, thanks!

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CLAs look good, thanks!

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[jamesdaniels]

Thanks!