updated doc string to show `NOT (EXISTS` instead of `NOT EXISTS`, as …

updated doc string to show `NOT (EXISTS` instead of `NOT EXISTS`, as …

…that is what SqlAlchemy now renders.

SqlAlchemy currently renders `NOT (EXISTS` not `NOT EXISTS`. This fixes the example SQL.

### Description
The change is a docstring only.

### Checklist

This pull request is:

- [x] A documentation / typographical 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: #4593
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4593
Pull-request-sha: fef577313242a1f79feb18969118a8fbb93bb6e9

Change-Id: Iee826a1754d76c93c014b44374e0f9aeabb8d0b6