[wp] Fixes Changelog dates

src/plugins/wp/Changelog

@@ -28,14 +28,14 @@ Plugin WP <next-release>
 Plugin WP 23.0 (Vanadium)
 #########################
 
-- WP          [2020-03-01] Section « Limitation & Roadmap » added to the WP manual.
-- WP          [2020-03-01] New internal WP engine, fixing many issues related to
+- WP          [2021-03-01] Section « Limitation & Roadmap » added to the WP manual.
+- WP          [2021-03-01] New internal WP engine, fixing many issues related to
                            control flow graph and local variable scoping.
                            Support for stmt contracts has been removed.
                            Support for looping gotos has been removed.
                            Altough unsound, the legacy engine is still
                            accessible via -wp-legacy option.
-- WP          [2020-01-25] Improved -wp-unfold-assigns <depth>
+- WP          [2021-01-25] Improved -wp-unfold-assigns <depth>
                            Now recursively applies to all compounds
 -* WP         [2020-01-20] Fixes opaque structures handling
 - TIP         [2020-11-06] New tactic: Sequence unrolling