Don't forget to build the howto-index file.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/Makefile

@@ -4,6 +4,7 @@ MAN7_TXT=git.txt
 DOC_HTML=$(patsubst %.txt,%.html,$(MAN1_TXT) $(MAN7_TXT))
 DOC_HTML += glossary.html
 DOC_HTML += tutorial.html
+DOC_HTML += howto-index.html
 DOC_MAN1=$(patsubst %.txt,%.1,$(MAN1_TXT))
 DOC_MAN7=$(patsubst %.txt,%.7,$(MAN7_TXT))