Correct 'Session.is_modified' method documentation

Correct 'Session.is_modified' method documentation

Correct 'Session.is_modified' method documentation.

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### Description
Correct documentation according to discussion: https://github.com/sqlalchemy/sqlalchemy/discussions/11481#discussioncomment-9759171

### 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: #11501
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/11501
Pull-request-sha: 16e949af544b4a1a94ccc5660e164fd2bca8be78

Change-Id: Ia8be0e3865190f0667de006c3006ecf646ef8422