Fix a typo

Author: wchen-r7

lib/msf/ui/console/command_dispatcher/db.rb

@@ -907,7 +907,7 @@ def cmd_vulns(*args)
       end
     end
 
-    if out_file
+    if output_file
       File.write(output_file, tbl.to_csv)
       print_status("Wrote vulnerability information to #{output_file}")
     end