git help config: s/insn/instruction/

"insn" appears to be an in-code abbreviation and should not appear
in manual/help pages.

Signed-off-by: Matthias Krüger <matthias.krueger@famsik.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'maint-1.8.1' into maint

* maint-1.8.1:
  git help config: s/insn/instruction/