Fix typo in agent/helpers.go
Fix typo in agent/helpers.go

Signed-off-by: NanXiao <11e623a37e87cf7995c466723ec99688d55cae8c@chinadtrace.org>

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