Update api-reference.md doc (#7065)
Update api-reference.md doc (#7065)

- moving description of coordinator isLeader endpoint
himanshug/druid,jon-wei/druid,gianm/druid,knoguchi/druid,nishantmonu51/druid,monetate/druid,Fokko/druid,monetate/druid,deltaprojects/druid,pjain1/druid,druid-io/druid,pjain1/druid,monetate/druid,implydata/druid,leventov/druid,jon-wei/druid,knoguchi/druid,Fokko/druid,gianm/druid,michaelschiff/druid,implydata/druid,mghosh4/druid,gianm/druid,michaelschiff/druid,monetate/druid,gianm/druid,deltaprojects/druid,nishantmonu51/druid,michaelschiff/druid,himanshug/druid,druid-io/druid,leventov/druid,druid-io/druid,deltaprojects/druid,mghosh4/druid,druid-io/druid,Fokko/druid,himanshug/druid,gianm/druid,pjain1/druid,Fokko/druid,nishantmonu51/druid,pjain1/druid,mghosh4/druid,nishantmonu51/druid,leventov/druid,mghosh4/druid,nishantmonu51/druid,mghosh4/druid,deltaprojects/druid,Fokko/druid,nishantmonu51/druid,deltaprojects/druid,monetate/druid,jon-wei/druid,knoguchi/druid,deltaprojects/druid,implydata/druid,knoguchi/druid,himanshug/druid,Fokko/druid,mghosh4/druid,druid-io/druid,implydata/druid,jon-wei/druid,implydata/druid,monetate/druid,pjain1/druid,gianm/druid,nishantmonu51/druid,knoguchi/druid,michaelschiff/druid,michaelschiff/druid,implydata/druid,leventov/druid,pjain1/druid,jon-wei/druid,deltaprojects/druid,mghosh4/druid,jon-wei/druid,gianm/druid,leventov/druid,pjain1/druid,himanshug/druid,monetate/druid,michaelschiff/druid,jon-wei/druid,michaelschiff/druid,Fokko/druid