Fixed Pandoc warning

I was using --chapter, which is deprecated by PanDoc.

pdf/book.tplx

@@ -31,6 +31,6 @@
 ((* endblock predoc *))
 
 ((* block markdowncell scoped *))
-((( cell.source | citation2latex | strip_files_prefix | convert_pandoc('markdown+tex_math_double_backslash', 'json',extra_args=[]) | resolve_references | convert_pandoc('json','latex', extra_args=["--chapters"]) )))
+((( cell.source | citation2latex | strip_files_prefix | convert_pandoc('markdown+tex_math_double_backslash', 'json',extra_args=[]) | resolve_references | convert_pandoc('json','latex', extra_args=["--top-level-division=chapter"]) )))
 ((* endblock markdowncell *))