docs: fix another typo in docker-build man page
docs: fix another typo in docker-build man page

s/arbtrary/arbitrary

Signed-off-by: Chen Hanxiao <818f8a165c25fa146ecad28e2ef3e3fb996fa364@cn.fujitsu.com>

josemonsalve2/docker,carlye566/docker_eric,carlye566/docker_eric,ClusterHQ/docker-plugins,seanmcdaniel/docker,josemonsalve2/docker,AdamOssenford/docker-pi,ClusterHQ/docker-plugins,seanmcdaniel/docker,carlye566/docker_eric,AdamOssenford/docker-pi,josemonsalve2/docker,ClusterHQ/docker-plugins,seanmcdaniel/docker,AdamOssenford/docker-pi