Fixed XML docs for netstandard1.6 build
Fixed XML docs for netstandard1.6 build

mikkelbu/nunit,NikolayPianikov/nunit,appel1/nunit,OmicronPersei/nunit,JustinRChou/nunit,nunit/nunit,mjedrzejek/nunit,nunit/nunit,JustinRChou/nunit,appel1/nunit,mikkelbu/nunit,NikolayPianikov/nunit,OmicronPersei/nunit,mjedrzejek/nunit