use expand-file-name rather than concat with org-directory

[emacs18]

I would like to suggest that the setup instruction be modified. Rather than

(concat org-directory "notes.org")

I think following should be used

(expand-file-name "notes.org" org-directory)

The latter works whether or not the value of org-directory has trailing slash.
On my emacs 27, the default value of this is "~/org" without a trailing slash.