notes.h: fix typos in comment

notes.h: fix typos in comment

In 1d7297513d (notes: break set_display_notes() into smaller functions,
2019-12-11), we introduced a comment which had a couple of typos. In the
first typo, we referenced 'enable_default_display_notes' instead of
'enable_ref_display_notes'. In the second typo, we wrote "is a points to"
instead of "is a pointer to". Correct both of these typos.

Reported-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>