org-pcomplete: Fix completion for block boundaries

* lisp/org-pcomplete.el (pcomplete/org-mode/file-option): Remove
  spurious colons from block boundaries.

lisp/org-pcomplete.el

@@ -155,8 +155,8 @@ When completing for #+STARTUP, for example, this function returns
 	    (let (block-names)
 	      (mapc (lambda (block-name)
 		      (let ((name (car block-name)))
-			(push (format "END_%s: " name) block-names)
-			(push (format "BEGIN_%s: " name) block-names)
+			(push (format "END_%s " name) block-names)
+			(push (format "BEGIN_%s " name) block-names)
 			(push (format "ATTR_%s: " name) block-names)))
 		    org-element-block-name-alist)
 	      block-names)