Update ttl_expiration_expression example

[ianjevans]

Fixes DOC-8267.

• [ ] Content review
• [ ] Backport to supported versions

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Files changed:

• src/current/v23.2/row-level-ttl.md

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[rmloveland]

hey @dikshant any objection to merging this old PR? nothing super urgent but might as well close it out / clean this up

unless you think the expiration expression should not have a default value - idk

[rmloveland]

Closing since this work is part of https://github.com/cockroachdb/docs/pull/18605