fix typo in readme
fix typo in readme

Signed-off-by: Victor Vieux <ce584ac70734cfa5a848cd28039798932c7ea675@docker.com>

elifa/swarmkit,RenaudWasTaken/swarmkit,cezarsa/swarmkit,RenaudWasTaken/swarmkit,runshenzhu/swarmkit,deljuven/swarmkit,jlhawn/swarmkit,vasil-yordanov/swarmkit,deljuven/swarmkit,docker/swarmkit,sungwonh/swarmkit,cheyang/swarmkit,AliyunContainerService/swarmkit,fate-grand-order/swarmkit,vasil-yordanov/swarmkit,allencloud/swarmkit,yp-engineering/swarmkit,aaronlehmann/swarmkit,jlhawn/swarmkit,Noyan759/swarmkit,runshenzhu/swarmkit,aaronlehmann/swarmkit,yongtang/swarmkit,diogomonica/swarmkit,dperny/swarmkit-1,fate-grand-order/swarmkit,allencloud/swarmkit,alyyousuf7/swarmkit,elifa/swarmkit,Noyan759/swarmkit,alyyousuf7/swarmkit,cheyang/swarmkit,yp-engineering/swarmkit,diogomonica/swarmkit,dperny/swarmkit-1,cezarsa/swarmkit,yongtang/swarmkit,sungwonh/swarmkit,docker/swarmkit,AliyunContainerService/swarmkit