doc: Update Japanese lxc-create(1) for --thinpool option

Update for commit 62c70ee2c1b49e7ecd2bfe156a66cbcfc5ecb502

Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>