Fix indentation in docstring
Fix indentation in docstring

The incorrect indentation causes improper formatting when the docs
are published.

Signed-off-by: Erik Johnson <43d4bedced1af4597739798c38c6ae5eac3c296e@gmail.com>

vpetersson/docker-py,docker/docker-py,vpetersson/docker-py,funkyfuture/docker-py,vdemeester/docker-py,youhong316/docker-py,vdemeester/docker-py,funkyfuture/docker-py,docker/docker-py,youhong316/docker-py