CONTRIBUTING.md: fix BG_DRIVER / BG_CACHE for carbon.conf
CONTRIBUTING.md: fix BG_DRIVER / BG_CACHE for carbon.conf

Thib17/biggraphite,iksaif/biggraphite,iksaif/biggraphite,criteo/biggraphite,Thib17/biggraphite,Thib17/biggraphite,iksaif/biggraphite,dpanth3r/biggraphite,criteo/biggraphite,criteo/biggraphite,criteo/biggraphite,iksaif/biggraphite,Thib17/biggraphite