notmuch-maildir-fcc: elisp syntax fixes

1)use insert-buffer-substring

Rather than the insert-buffer. Emacs complains that it is for interactive use
and not for use within elisp. So use insert-buffer-substring which does the
same thing when not handed any 'begin' 'end' parameters.

2)replace caddr with (car (cdr (cdr)))

The former requires 'cl to be loaded and during make install emacs complained
about not knowing it.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>