multi-pack-index: correct configuration in documentation

It's core.multiPackIndex, not pack.multiIndex.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/multi-pack-index.txt

@@ -36,7 +36,7 @@ Design Details
   directory of an alternate. It refers only to packfiles in that
   same directory.
 
-- The pack.multiIndex config setting must be on to consume MIDX files.
+- The core.multiPackIndex config setting must be on to consume MIDX files.
 
 - The file format includes parameters for the object ID hash
   function, so a future change of hash algorithm does not require