fix: use correct path for /etc/default file

/etc/defaults was an invalid path.

contrib/dokku-update

@@ -2,9 +2,9 @@
 set -eo pipefail
 shopt -s nullglob
 
-if [[ -r /etc/defaults/dokku ]]; then
+if [[ -r /etc/default/dokku ]]; then
   # shellcheck disable=SC1091
-  source /etc/defaults/dokku
+  source /etc/default/dokku
 fi
 
 export DOKKU_ROOT=${DOKKU_ROOT:=~dokku}