test_detect_ref_format: fix comment

When 58aaf59 (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>

Author: dscho

t/test-lib-functions.sh

@@ -1724,7 +1724,7 @@ test_detect_hash () {
 	esac
 }
 
-# Detect the hash algorithm in use.
+# Detect the ref format in use.
 test_detect_ref_format () {
 	echo "${GIT_TEST_DEFAULT_REF_FORMAT:-files}"
 }