socket-activate: fix comment

"optargs" is not a thing.

src/socket-activate/socket-activate.c

@@ -422,7 +422,7 @@ static int parse_argv(int argc, char *argv[], char ***remaining_args) {
                                         log_warning("File descriptor name \"%s\" is not valid.", esc);
                                 }
 
-                        /* Empty optargs means one empty name */
+                        /* Empty argument means one empty name */
                         r = strv_extend_strv(&arg_fdnames,
                                              strv_isempty(names) ? STRV_MAKE("") : names,
                                              false);