t/t5319-multi-pack-index.sh: fix copy-and-paste error in t5319.39

Commit d4bf1d88b90 (multi-pack-index: verify missing pack, 2018-09-13) adds a new test to the MIDX test script to test how we handle missing packs. While the commit itself describes the test as "verify missing pack[s]", the test itself is actually called "verify packnames out of order", despite that not being what it tests. Likely this was a copy-and-paste of the test immediately above it of the same name. Correct this by renaming the test to match the commit message. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>