test_detect_ref_format: fix comment

When 58aaf59133b (t: introduce GIT_TEST_DEFAULT_REF_FORMAT envvar,
2023-12-29) copy-edited the `test_detect_hash` function, the code
comment was accidentally left unchanged. Let's adjust it.

Noticed-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 78e054a..10bbcea 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1702,7 +1702,7 @@
 	esac
 }
 
-# Detect the hash algorithm in use.
+# Detect the ref format in use.
 test_detect_ref_format () {
 	echo "${GIT_TEST_DEFAULT_REF_FORMAT:-files}"
 }