bug#24738: 26.0.50; Incorrect Edebug spec for seq-let

[Gemini Lasswell]

The Edebug spec for seq-let is incorrect, causing code that uses it to
not work correctly while instrumented for debugging.

To reproduce:

1. emacs -Q
2. enter the following code into *scratch*:

(defun seq-let-bug ()
  (let ((nums [1 2 3 4 5]))
    (seq-let (a b c) nums
      (message "a=%s b=%s c=%s" a b c))))

3. C-u C-M-x
4. M-: (seq-let-bug) RET

Result: a=nil b=nil c=nil
Expected result: a=1 b=2 c=3

Changing seq-let's edebug spec to (debug (sexp form body))
fixes this for me.