aerospace: use upstream example for exec-on-workspace-change (#7765)

White technically any example would do, it's better to use the one from
upstream docs, otherwise we may confuse users trying to repeat upstream
aerospace sketchybar plugin recipe. (I for one fell a victim of the
confusion...)

https://nikitabobko.github.io/AeroSpace/goodies#show-aerospace-workspaces-in-sketchybar

modules/programs/aerospace.nix

@@ -255,7 +255,7 @@ in
             example = [
               "/bin/bash"
               "-c"
-              "sketchybar --trigger aerospace_workspace_change FOCUSED=$AEROSPACE_FOCUSED_WORKSPACE"
+              "sketchybar --trigger aerospace_workspace_change FOCUSED_WORKSPACE=$AEROSPACE_FOCUSED_WORKSPACE"
             ];
             description = "Commands to run every time workspace changes.";
           };