Documentation: Fix howto/revert-branch-rebase.html generation

The rule for howto/*.html used "$?", which expands to the list of all
newer prerequisites, including asciidoc.conf added by another rule.
"$<" should be used instead.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/Makefile

@@ -107,7 +107,7 @@ WEBDOC_DEST = /pub/software/scm/git/docs
 
 $(patsubst %.txt,%.html,$(wildcard howto/*.txt)): %.html : %.txt
 	rm -f $@+ $@
-	sed -e '1,/^$$/d' $? | asciidoc -b xhtml11 - >$@+
+	sed -e '1,/^$$/d' $< | asciidoc -b xhtml11 - >$@+
 	mv $@+ $@
 
 install-webdoc : html