README: fix typo
README: fix typo

Signed-off-by: Johan Euphrosine <aeff70c2aef2cec1cd2754756b5d3e6402001c31@google.com>
elifa/swarmkit,elifa/swarmkit,allencloud/swarmkit,cezarsa/swarmkit,sungwonh/swarmkit,RenaudWasTaken/swarmkit,AliyunContainerService/swarmkit,deljuven/swarmkit,aaronlehmann/swarmkit,fate-grand-order/swarmkit,fate-grand-order/swarmkit,dperny/swarmkit-1,docker/swarmkit,vasil-yordanov/swarmkit,Noyan759/swarmkit,AliyunContainerService/swarmkit,cezarsa/swarmkit,sungwonh/swarmkit,diogomonica/swarmkit,RenaudWasTaken/swarmkit,runshenzhu/swarmkit,alyyousuf7/swarmkit,vasil-yordanov/swarmkit,jlhawn/swarmkit,yp-engineering/swarmkit,deljuven/swarmkit,yongtang/swarmkit,aaronlehmann/swarmkit,allencloud/swarmkit,yp-engineering/swarmkit,alyyousuf7/swarmkit,cheyang/swarmkit,runshenzhu/swarmkit,dperny/swarmkit-1,docker/swarmkit,yongtang/swarmkit,diogomonica/swarmkit,cheyang/swarmkit,Noyan759/swarmkit,jlhawn/swarmkit