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

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.
