doc: Fix syntax errors in sample Emacs config

Signed-off-by: Diego Biurrun <diego@biurrun.de>

doc/developer.texi

@@ -235,8 +235,8 @@ For Emacs, add these roughly equivalent lines to your @file{.emacs.d/init.el}:
 (c-add-style "libav"
              '("k&r"
                (c-basic-offset . 4)
-               (indent-tabs-mode nil)
-               (show-trailing-whitespace t)
+               (indent-tabs-mode . nil)
+               (show-trailing-whitespace . t)
                (c-offsets-alist
                 (statement-cont . (c-lineup-assignments +)))
                )