Correct plugin example

$APP is not available until after the if guards

docs/plugins.md

@@ -39,7 +39,7 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 case "$1" in
   hello)
     [[ -z $2 ]] && echo "Please specify an app to run the command on" && exit 1
-    [[ ! -d "$DOKKU_ROOT/$APP" ]] && echo "App $APP does not exist" && exit 1
+    [[ ! -d "$DOKKU_ROOT/$2" ]] && echo "App $2 does not exist" && exit 1
     APP="$2";
 
     echo "Hello $APP"