fix(_slackpkg): do not scan after cword

Author: akinomyoga

completions/_slackpkg

@@ -51,7 +51,7 @@ _comp_cmd_slackpkg()
     . "$config"
 
     local i action
-    for ((i = 1; i < ${#words[@]}; i++)); do
+    for ((i = 1; i < cword; i++)); do
         if [[ ${words[i]} != -* ]]; then
             action="${words[i]}"
             break