Add :terrain to changelog

changelog.md

@@ -60,6 +60,7 @@
      attributes should now by default changed via [effect].
    * New tag [modify_unit_type] that goes into [campaign]/[era]/[modification] and can change some
      unit type properties like advancement and recruit costs.
+   * New debug command :terrain for changing the terrain of the selected hex (PR#4405)
  ### Miscellaneous and bug fixes
    * Fixed :droid's arguments not all being optional (Issue#4308)
    * Ported the "expand-terrain-macros", "wmlflip", "wmlparser", "umc-dev/build/update_version",