config.txt: correct the note about uploadpack.packObjectsHook

config.txt: correct the note about uploadpack.packObjectsHook

Document for uploadpack.packObjectsHook is added in [1] and consists
of two paragraphs, the second one is quite important about where this
variable can stay.

When the paragraph about uploadpack.allowFilter is added in [2], it's
added in between the two paragraphs. This makes the "this is non-repo
level config" note incorrectly apply to allowFilter instead of
packObjectsHook. Move allowFilter paragraph down to fix this.

[1] 20b20a22f8 (upload-pack: provide a hook for running pack-objects -
    2016-05-18)

[2] 10ac85c785 (upload-pack: add object filtering for partial clone -
    2017-12-08)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>