docs: Revise update_table_expiration sample

[thejaredchapman]

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Towards #790 🦕

[snippet-bot]

Here is the summary of changes.

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• samples/snippets/update_table_expiration.py:22, tag bigquery_update_table_expiration
• samples/snippets/update_table_expiration.py:39, tag bigquery_update_table_expiration

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[tswast]

Looks good! But let's update docs/snippets.py with a comment that we should delete the duplicate sample there once we update cloud.google.com.

[tswast]

@thejaredchapman I think we might want to rebase this just so we can fixup the CLA check.

Would love to see the commit for #1516 as part of this PR instead of as a second PR, too.

[meredithslota]

Manually applied the cla: yes label since it's related to a Xoogler contribution made during employment.

edit This didn't work, ha — someone will need to click the "request exception" link in the CLA checker log.

[tswast]

I've squashed all commits and re-pushed. I believe that should resolve the CLA issue.