change language specifier for some of the markdown fenced code blocks
change language specifier for some of the markdown fenced code blocks

because both "extempore" and "xtlang" work, and with "extempore" I get pretty
syntax colouring when editing the md files in Emacs. And that's nice.

extemporelang/extemporelang.github.io,extemporelang/extemporelang.github.io,extemporelang/extemporelang.github.io