Fix contacts complete not working problem

> I found ~org-contacts-org-complete-function~ returned a special value:
>
> #+begin_example
> #f(compiled-function (string pred action) #<bytecode -0x9e1a398d61d3acb>)
> #+end_example

I can't see any way M-: (org-contacts-org-complete-function) RET
can return the above value.  So I suspect a "pilot error".
This looks like the 3rd value in the returned list (i.e. the value
returned by `completion-table-dynamic`).

> #+begin_src emacs-lisp
> (defun org-contacts-org-complete-function ()
>   "Function used in `completion-at-point-functions' in `org-mode' to complete @name."
>   (when-let* ((bounds (bounds-of-thing-at-point 'symbol))
>               (begin (1- (car bounds)))
>               (end (cdr bounds))
>               (symbol (buffer-substring-no-properties begin end))
>               (org-contacts-prefix-p (string-prefix-p "@" symbol))
>               ;; (prefix (substring-no-properties symbol 1 nil))
>               )
>     (when org-contacts-prefix-p
>       (list begin
>             end
>             (completion-table-dynamic
>              (lambda (_)
>                (mapcar
>                 (lambda (contact) (plist-get contact :name))
>                 (org-contacts--all-contacts))))))))
> #+end_src

This gives a `begin..end` region which presumably includes `@`.
Does (plist-get contact :name) return names that start with `@`?
If not, the completion will never match.

> And test with execute following ~add-hook~ in org-mode buffer or
>  emacs-lisp-mode buffer:

In emacs-lisp-mode, `@` has symbol syntax, so
(bounds-of-thing-at-point 'symbol)
will include `@` in the returned region, whereas in Org mode
`@` seems to have punctuation syntax so the `@` will not be included in
the returned region.

Maybe instead of `bounds-of-thing-at-point` you want to use something
less "magic", like (skip-chars-backward "[:alnum:]@").

> #+begin_src emacs-lisp
> (add-hook 'completion-at-point-functions 'org-contacts-org-complete-function nil 'local)
> #+end_src

org-contacts.el

@@ -698,7 +698,7 @@ description."
             (completion-table-dynamic
              (lambda (_)
                (mapcar
-                (lambda (contact) (plist-get contact :name))
+                (lambda (contact) (concat "@" (plist-get contact :name)))
                 (org-contacts--all-contacts))))
 
             ;; :predicate 'stringp