docs: move gitremote-helpers into section 7

docs: move gitremote-helpers into section 7

It is currently in section 1, but that section is intended for
"Executable programs or shell commands".

A more appropriate place is section 7: "Miscellaneous (including macro
packages and conventions), e.g. man(7), groff(7)".

This issue should have been detected earlier by `make check-docs`, but
was missed due to a bug in that Makefile target (that we are about to
fix).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>