gptel: tool block spacing changes (#735)

gptel.el (gptel--display-tool-results): Add missing newline after
`#+end_tool' marker.  This is part of a set of changes intended to fix
whitespace handling around the insertion of tool result blocks in
buffers.

Author: LuciusChen

gptel.el

@@ -3118,7 +3118,7 @@ for tool call results.  INFO contains the state of the request."
                     (propertize
                      (concat "\n" call "\n\n" (org-escape-code-in-string result))
                      'gptel `(tool . ,id))
-                    "\n#+end_tool")
+                    "\n#+end_tool\n")
                  ;; TODO(tool) else branch is handling all front-ends as markdown.
                  ;; At least escape markdown.
                  (concat