doc: developer: Allow tabs in the vim configuration for Automake files

While we do not use Automake in libav, this allows our config to be
used more globally without introducing unwanted breakage.

doc/developer.texi

@@ -229,7 +229,7 @@ set softtabstop=4
 set cindent
 set cinoptions=(0
 " Allow tabs in Makefiles.
-autocmd FileType make set noexpandtab shiftwidth=8 softtabstop=8
+autocmd FileType make,automake set noexpandtab shiftwidth=8 softtabstop=8
 " Trailing whitespace and tabs are forbidden, so highlight them.
 highlight ForbiddenWhitespace ctermbg=red guibg=red
 match ForbiddenWhitespace /\s\+$\|\t/