Documentation: build technical/multi-pack-index

The git-multi-pack-index doc links to technical/multi-pack-index.html.
Ensure it is built to prevent a broken link.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -73,6 +73,7 @@ TECH_DOCS += technical/hash-function-transition
 TECH_DOCS += technical/http-protocol
 TECH_DOCS += technical/index-format
 TECH_DOCS += technical/long-running-process-protocol
+TECH_DOCS += technical/multi-pack-index
 TECH_DOCS += technical/pack-format
 TECH_DOCS += technical/pack-heuristics
 TECH_DOCS += technical/pack-protocol