Documentation/gitformat-pack.txt: fix typo
e0d1bcf825 (multi-pack-index: add format details, 2018-07-12) describes
the MIDX's "PNAM" chunk as having entries which are "null-terminated
strings".
This is a typo, as strings are terminated with a NUL character, which is
a distinct concept from "NULL" or "null", which we typically reserve for
the void pointer to address 0.
Correct the documentation accordingly.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>