TOC for Markdown should now work for both Pandoc < 2.0 and >= 2.0

The use of $toc$ and $table-of-contents$ flags changed, see:
https://pandoc.org/releases.html#pandoc-2.0-29-oct-2017
under "Behaviour changes"

doc/gf-refman.md

@@ -2,7 +2,7 @@
 title: GF Language Reference Manual
 author: Aarne Ranta, Krasimir Angelov
 date: June 2014, GF 3.6
-toc: true
+show-toc: true
 ---
 
 This document is a reference manual to the GF programming language. GF,