docs: correct docs for LoadJobConfig.destination_table_description

[mgorsk1]

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Fixes #811.

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[mgorsk1]

@googlebot I signed it!

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[mgorsk1]

@googlebot I fixed it.

[plamut]

Thanks for spotting this and taking the initiative to fix it yourself! FWIW, I created an issue and linked it with this PR.

I believe the sentence should be re-worded, after which we can merge this.

[plamut]

Looks good, thanks again for your contribution!

[mgorsk1]

My pleasure! Thanks for prompt review.

[plamut]

Changed the PR title to make the commit message bot happy, no need to update the PR itself.

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