org.el: Don't add a column when there is only one tag offered for completion.

* org.el (org-set-tags): Don't add a column when there is only
one tag offered for completion.

lisp/org.el

@@ -13407,7 +13407,7 @@ With prefix ARG, realign all tags in headings in the current buffer."
 		     current-tags inherited-tags table
 		     (if org-fast-tag-selection-include-todo
 			 org-todo-key-alist))
-		  (let ((org-add-colon-after-tag-completion t))
+		  (let ((org-add-colon-after-tag-completion (< 1 (length table))))
 		    (org-trim
 		     (org-icompleting-read "Tags: "
 					   'org-tags-completion-function