t2107: fix mention of the_index.cache_changed

Commit [1] added a test to t2107-update-index-basic.sh with a comment
that mentions macro "active_cache_changed".  Later in [2], the macro was
removed and its usage in function cmd_update_index in file
builtin/update-index.c was replaced with "the_index.cache_changed".

Fix the outdated comment in file t2107-update-index-basic.sh.

[1] fa137f67a4 (lockfile.c: store absolute path, 2014-11-02)
[2] dc594180d9 (cocci & cache.h: apply variable section of "pending"
    index-compatibility, 2022-11-19)

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2107-update-index-basic.sh

@@ -83,7 +83,7 @@ test_expect_success '.lock files cleaned up' '
 	cd repo &&
 	git config core.worktree ../../worktree &&
 	# --refresh triggers late setup_work_tree,
-	# active_cache_changed is zero, rollback_lock_file fails
+	# the_index.cache_changed is zero, rollback_lock_file fails
 	git update-index --refresh --verbose >out &&
 	test_must_be_empty out &&
 	! test -f .git/index.lock