Fix help-form binding in dired-create-files

This fixes Bug#32630: since "dired-aux" moved to lexical binding mode,
the free variable TO in the constructed HELP-FORM got out of scope of
the surrounding 'let'.

* lisp/dired-aux.el (dired-create-files): Make the binding of
HELP-FORM a string.

lisp/dired-aux.el

@@ -1712,7 +1712,7 @@ or with the current marker character if MARKER-CHAR is t."
           (let* ((overwrite (file-exists-p to))
                  (dired-overwrite-confirmed ; for dired-handle-overwrite
                   (and overwrite
-                       (let ((help-form '(format-message "\
+                       (let ((help-form (format-message "\
 Type SPC or `y' to overwrite file `%s',
 DEL or `n' to skip to next,
 ESC or `q' to not overwrite any of the remaining files,