Include PostgreSQL in trigger test and correct documentation example

Include PostgreSQL in trigger test and correct documentation example

Include PostgreSQL dialect in trigger test and correct DDL example in documentation

A user highlighted that the syntax in the DDL trigger example was incorrect for PostgreSQL. The trigger tests where also skipping the PostgreSQL dialect until the syntax was corrected. This PR fixes both of these issues.

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!**
Fixes: #4037
Closes: #5548
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5548
Pull-request-sha: 1db5e47adb90f9d51e247711dcfdbb274fb7bf73

Change-Id: I06edbcab99c82a3ce25581b81f8d2a4a028c07c3