fix broken link to message format docs
fix broken link to message format docs

Former-commit-id: 799ec57d26c557d72e9684dbf0006f47d340bcfd
MassTransit/MassTransit,SanSYS/MassTransit,SanSYS/MassTransit,jacobpovar/MassTransit,phatboyg/MassTransit,MassTransit/MassTransit,jacobpovar/MassTransit,phatboyg/MassTransit