Makefile: fix typo in 'docker-test-proxy'
Makefile: fix typo in 'docker-test-proxy'

Signed-off-by: Gyu-Ho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

vimalk78/etcd,xiang90/etcd,bluebreezecf/etcd,glevand/coreos--etcd,bluebreezecf/etcd,mkumatag/etcd,lorneli/etcd,glevand/coreos--etcd,mqliang/etcd,etcd-io/etcd,dannysauer/etcd,mqliang/etcd,akauppi/etcd,sinsharat/etcd,bdarnell/etcd,fanminshi/etcd,coreos/etcd,cockroachdb/etcd,lorneli/etcd,etcd-io/etcd,mqliang/etcd,fanminshi/etcd,heyitsanthony/etcd,whuwxl/etcd,bdarnell/etcd,ajityagaty/etcd,rnaveiras/etcd,sebrandon1/etcd,whuwxl/etcd,xiang90/etcd,li-ang/etcd,cockroachdb/etcd,gyuho/etcd,rnaveiras/etcd,sinsharat/etcd,joshix/etcd,coreos/etcd,ajityagaty/etcd,vimalk78/etcd,sebrandon1/etcd,dannysauer/etcd,joshix/etcd,ajityagaty/etcd,gyuho/etcd,heyitsanthony/etcd,li-ang/etcd,rnaveiras/etcd,etcd-io/etcd,mkumatag/etcd,mkumatag/etcd,akauppi/etcd