Adding special character caveat to PG DBM doc

Customers often get password validation failed errors from our postgres integration/DBM which is often caused by special characters. The line I added is copied directly from the MySQL version of this doc: https://github.com/DataDog/documentation/edit/master/content/en/database_monitoring/setup_mysql/rds.md/

content/en/database_monitoring/setup_postgres/selfhosted.md

@@ -248,6 +248,8 @@ Installing the Datadog Agent also installs the Postgres check which is required
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+**Note**: Wrap your password in single quotes in case a special character is present.
+
 2. [Restart the Agent][10].
 
 ### Collecting logs (optional)