docs(CHANGES): Note libtmux update

Author: tony

CHANGES

@@ -31,8 +31,10 @@ $ pipx install --suffix=@next 'tmuxp' --pip-args '\--pre' --force
   - Clean up CodeQL
 
 - Bump poetry 1.1.x -> 1.2.x
-- Bump libtmux 0.15.1 -> 0.15.3 (#821, pytest plugin improvements, root-level
-  conftest.py)
+- Bump libtmux 0.15.1 -> 0.15.6
+  - 0.15.6 (#823, pytest fixes, packaging improvements)
+  - 0.15.3 (#821, pytest plugin improvements, root-level
+    conftest.py)
 
 ### Packaging