Update `table_per_related` example for Declarative API

Update `table_per_related` example for Declarative API

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### Description
When reading more of the Generic Associations, I found that the examples state "Annotated example", and wondered what that meant, since in other parts of the docs that means they are using the new 2.0 style.

I tried to update this example to be more in line with the new style, including a little f-string update.

I completely understand this is unlikely to be merged as-is - but wanted to understand more about the right way to use modern styles to properly code well-hinted, more "exotic" implementations.

Outstanding questions:
- Should examples pass pass `mypy --strict` ? It doesn't right now.
- Are there better ways to apply `Mapped` within the `type(...)` definition, so we could skip importing `Integer`?

### Checklist
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This pull request is:

(Do non-running examples count as code fixes/features?)

- [x] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed

**Have a nice day!**

Closes: #10450
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10450
Pull-request-sha: a6a54d69487429c8b3a7758cf22913e6a2156316

Change-Id: Ie7c7526daed5bf907d41f666459759b9a986b8c2