fix config option type in readme
fix config option type in readme

priestjim/relx,bsmr-erlang/relx,erlware/relx,lrascao/relx,syl20bnr/relx,tsloughter/relx,ingwinlu/relx,zugolosian/relx,timclassic/relx,RJ/relx,Licenser/relx,umatomba/relx,ethercrow/relx,bitwalker/relx