Fix syntax in README
Fix syntax in README

Fenced code blocks were not being rendered properly due to an indentation error.
elliotjo/marti_common,elliotjo/marti_common,elliotjo/marti_common,evenator/marti_common,swri-robotics/marti_common,elliotjo/marti_common,evenator/marti_common,swri-robotics/marti_common,swri-robotics/marti_common,evenator/marti_common,swri-robotics/marti_common,evenator/marti_common