doc: fix spelling instance-exec.md
doc: fix spelling instance-exec.md

Signed-off-by: Ruth Fuchss <90c286bb8b7c8291b9f5a2a1d38dc233728381fe@canonical.com>

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