.dir-locals.el: Set indent-tabs-mode to nil in Elisp sources

Despite .dir-locals.el having a catchall value of t for
indent-tabs-mode, Org's Elisp files are a mix of tabs and spaces.
Emacs has used a value of nil for indent-tabs-mode since 93d4412046
(Set indent-tabs-mode to nil in (most) Elisp sources, 2015-04-15).  Do
the same.

Ref: https://orgmode.org/list/87eejsg9vw.fsf@gnu.org

.dir-locals.el

@@ -6,6 +6,8 @@
   (tab-width . 8)
   (fill-column . 70)
   (sentence-end-double-space . t))
+ (emacs-lisp-mode
+  (indent-tabs-mode))
  (org-mode
   (indent-tabs-mode)
   (org-adapt-indentation)