Documentation: fix version numbering

Version numbers in asciidoc-generated content (such as man pages)
went missing as of da8a366 (Documentation: refactor common operations
into variables).  Fix by putting the underscore back in the variable
name.

Signed-off-by: Sven van Haastregt <svenvh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -103,7 +103,7 @@ ASCIIDOC_HTML = xhtml11
 ASCIIDOC_DOCBOOK = docbook
 ASCIIDOC_CONF = -f asciidoc.conf
 ASCIIDOC_COMMON = $(ASCIIDOC) $(ASCIIDOC_EXTRA) $(ASCIIDOC_CONF) \
-		-agit-version=$(GIT_VERSION)
+		-agit_version=$(GIT_VERSION)
 TXT_TO_HTML = $(ASCIIDOC_COMMON) -b $(ASCIIDOC_HTML)
 TXT_TO_XML = $(ASCIIDOC_COMMON) -b $(ASCIIDOC_DOCBOOK)
 MANPAGE_XSL = manpage-normal.xsl