Fix CHANGELOG for version 1.3.1.

CHANGELOG.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+
+## [1.3.1] - 2023-01-14
 ### Fixed
  - Solvers now keep an reference to the added preconditioner to make sure the preconditioner
    is not finalized before the solver. This fixes crashes (segfaults) that could happen in
@@ -61,4 +63,5 @@ Initial release of HYPRE.jl.
 [1.1.0]: https://github.com/fredrikekre/HYPRE.jl/compare/v1.0.0...v1.1.0
 [1.2.0]: https://github.com/fredrikekre/HYPRE.jl/compare/v1.1.0...v1.2.0
 [1.3.0]: https://github.com/fredrikekre/HYPRE.jl/compare/v1.2.0...v1.3.0
-[Unreleased]: https://github.com/fredrikekre/HYPRE.jl/compare/v1.3.0...HEAD
+[1.3.1]: https://github.com/fredrikekre/HYPRE.jl/compare/v1.3.0...v1.3.1
+[Unreleased]: https://github.com/fredrikekre/HYPRE.jl/compare/v1.3.1...HEAD