[docs] Corrected 5.1 Postgres min version

Author: mickhawkins

general/releases/5.1.md

@@ -30,7 +30,7 @@ Moodle supports the following database servers. Again, version numbers are just
 
 | Database | Minimum version | Recommended |
 | --- | --- | --- |
-| [PostgreSQL](http://www.postgresql.org/) | 14 (increased since Moodle 4.5) | Latest |
+| [PostgreSQL](http://www.postgresql.org/) | 15 (increased since Moodle 5.0) | Latest |
 | [MySQL](http://www.mysql.com/) | 8.4 (increased since Moodle 4.5) | Latest |
 | [MariaDB](https://mariadb.org/) | 10.11.0 (increased since Moodle 4.5) | Latest |
 | [Microsoft SQL Server](http://www.microsoft.com/en-us/server-cloud/products/sql-server/) | 2017 | Latest |