client.go: fix golint
client.go: fix golint

    Line 66: warning: comment on exported method Client.InvokeWithSandbox should be of the form "InvokeWithSandbox ..." (golint)

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

containerd/nri