Fix readme.md documentation for the 'volume' arg in create_container

Readme.md mentions that creating a volume which can be rebound later is done via `volumes={"/some/mount/point": ""}`. This is incorrect, and doing so will cause the following error to be returned by the Remote API:

    500 Server Error: Internal Server Error ("create: ExportEnv json: cannot unmarshal string into Go value of type struct {}")

The correct syntax is `volumes={"/some/mount/point": {}}`.