Fix links broken by non-standard version.
Using version 2.15.1 fixed the duplicate tarball problem but broke the auto-generated links.  Fix them manually since this should not be a common problem.
Reported by Mohamad El-Rifai.
pgBackRest aims to be a simple, reliable backup and restore solution that can seamlessly scale up to the largest databases and workloads by utilizing algorithms that are optimized for database-specific requirements.
-pgBackRest [v2.15](https://github.com/pgbackrest/pgbackrest/releases/tag/release/2.15) is the current stable release. Release notes are on the [Releases](http://www.pgbackrest.org/release.html) page.
+pgBackRest [v2.15](https://github.com/pgbackrest/pgbackrest/releases/tag/release/2.15.1) is the current stable release. Release notes are on the [Releases](http://www.pgbackrest.org/release.html) page.
Documentation for v1 can be found [here](http://www.pgbackrest.org/1). No further releases are planned for v1 because v2 is backward-compatible with v1 options and repositories.
