fix typo

acme-downloader.sh

@@ -49,11 +49,11 @@ check_version() {
         echo "in order to fetch and install the new version you can use the option: --update"
         echo ""
     else
-        if [ -n $CHECK ]; then
+        if [[ -n $CHECK ]]; then
             echo "you are running the latest version"
         fi
     fi
-    if [ -n $CHECK ]; then
+    if [[ -n $CHECK ]]; then
         exit 0
     fi
 }
@@ -166,7 +166,7 @@ TYPE=$(echo $TYPE | tr '[:lower:]' '[:upper:]')
 
 check_version $0
 
-if [ -n $UPDATE ]; then
+if [[ -n $UPDATE ]]; then
     curl $SCRIPT_URL -o $0
     UPDATE_STATUS=$?
     if [ $UPDATE_STATUS == 0 ]; then