Fix a warning due to a blank specified as _____ rather than ____

[kconner]

I was thrown off thinking I'd created an error with my previous change, when I'd only revealed it.

SBCL 2.3.2 says:

; in: LAMBDA ()
;     (LISP-KOANS.TEST:ASSERT-EQUAL LISP-KOANS.KOANS.BACKQUOTE::_____
;                                   `(LISP-KOANS.KOANS.BACKQUOTE::WORD
;                                     ,LISP-KOANS.KOANS.BACKQUOTE::WORD
;                                     ,LISP-KOANS.KOANS.BACKQUOTE::WORD
;                                     LISP-KOANS.KOANS.BACKQUOTE::WORD))
; --> LISP-KOANS.TEST::INTERNAL-ASSERT LAMBDA
; ==>
;   #'(LAMBDA () LISP-KOANS.KOANS.BACKQUOTE::_____)
;
; caught WARNING:
;   undefined variable: LISP-KOANS.KOANS.BACKQUOTE::_____
;
; compilation unit finished
;   Undefined variable:
;     LISP-KOANS.KOANS.BACKQUOTE::_____
;   caught 1 WARNING condition

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