asciidoctor-extensions.rb.in: add missing word

asciidoctor-extensions.rb.in: add missing word

Commit a38edab7c8 (Makefile: generate doc versions via GIT-VERSION-GEN,
2024-12-06) stopped providing an attribute value "Git $(GIT_VERSION)" to
asciidoc/Asciidoctor over the command line. Instead, we now provide the
attribute to asciidoc through a generated asciidoc.conf, where the value
is generated as "Git @GIT_VERSION@".

In the similar mechanism for Asciidoctor, we forgot the "Git" prefix.
Restore it.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>