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

Commit d4bf1d8 (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>

Author: ttaylorr

t/t5319-multi-pack-index.sh

@@ -517,7 +517,7 @@ test_expect_success 'verify packnames out of order' '
 		"pack names out of order"
 '
 
-test_expect_success 'verify packnames out of order' '
+test_expect_success 'verify missing pack' '
 	corrupt_midx_and_verify $MIDX_BYTE_PACKNAME_ORDER "a" $objdir \
 		"failed to load pack"
 '