t2107: fix mention of the_index.cache_changed

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Fix the outdated comment in file t2107-update-index-basic.sh.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