Fix quoting in gnus-buffer-configuration

* lisp/gnus/gnus-win.el (gnus-buffer-configuration): Include
shell-command-buffer-name value rather than symbol in pipe
configuration.  (Bug#39138, bug#45154)

lisp/gnus/gnus-win.el

@@ -68,7 +68,7 @@ used to display Gnus windows."
   :type 'boolean)
 
 (defvar gnus-buffer-configuration
-  '((group
+  `((group
      (vertical 1.0
 	       (group 1.0 point)))
     (summary
@@ -142,7 +142,7 @@ used to display Gnus windows."
     (pipe
      (vertical 1.0
 	       (summary 0.25 point)
-	       (shell-command-buffer-name 1.0)))
+               (,shell-command-buffer-name 1.0)))
     (bug
      (vertical 1.0
 	       (if gnus-bug-create-help-buffer '("*Gnus Help Bug*" 0.5))