asciidoctor-extensions.rb.in: add missing word

Commit a38edab (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>

Author: Martin Ågren

Documentation/asciidoctor-extensions.rb.in

@@ -32,7 +32,7 @@ module Git
           output = output.sub(/<refmiscinfo class="source">.*?<\/refmiscinfo>/, "")
           output = output.sub(/<refmiscinfo class="manual">.*?<\/refmiscinfo>/, "")
           new_tags = "" \
-            "<refmiscinfo class=\"source\">@GIT_VERSION@</refmiscinfo>\n" \
+            "<refmiscinfo class=\"source\">Git @GIT_VERSION@</refmiscinfo>\n" \
             "<refmiscinfo class=\"manual\">Git Manual</refmiscinfo>\n"
           output = output.sub(/<\/refmeta>/, new_tags + "</refmeta>")
         end