docs: fix no parameter connect string example (#1056)

postgresql-42.1.4 - DriverManager.getConnection("jdbc:postgresql:/") throws org.postgresql.util.PSQLException: FATAL: database "/" does not exist. The trailing slash is being interpreted as the database name. DriverManager.getConnection("jdbc:postgresql:") connects to the default port on localhost using the user's name as the database.

README.md

@@ -85,7 +85,7 @@ For more information you can read [the PgJDBC driver documentation](https://jdbc
The driver recognises JDBC URLs of the form:
```
jdbc:postgresql:database
jdbc:postgresql:/
jdbc:postgresql:
jdbc:postgresql://host/database
jdbc:postgresql://host/
jdbc:postgresql://host:port/database