doc: Capitalize candid in doc/api-extensions.md
doc: Capitalize candid in doc/api-extensions.md

Signed-off-by: Hiroaki Nakamura <d5f119c83b49083ce2220cbef65198e8f74dd9e4@gmail.com>

hallyn/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd