docs: fix type annotation in Self-Referential Many-to-Many Relationsh…

docs: fix type annotation in Self-Referential Many-to-Many Relationsh…

A type annotation was wrong in the [Self-Referential Many-to-Many Relationship](https://docs.sqlalchemy.org/en/20/orm/join_conditions.html#self-referential-many-to-many-relationship) code example.

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### Description

The type annotation was `right_nodes: Mapped[List["None"]]`. I changed it to `Node` since we refer to the ORM class we're looking at.

### Checklist
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This pull request is:

- [x] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #10686
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10686
Pull-request-sha: 7671898116f1b8850e5d8f3ff0f940450a7c1bf4

Change-Id: Iab1535c3d00747eb8c9e9a17aea50606febedbf9
(cherry picked from commit 7157d16e3ba521c119958a727af51790ebdf3f34)