README: Update `ignore_namespace_structure` to `enforce_namespace_structure`
Was changed in e9849032
Pain point encountered in workflow for StaticEquilibriumConstraint
:
https://github.com/RobotLocomotion/drake/issues/14827
Was changed in e9849032
Pain point encountered in workflow for StaticEquilibriumConstraint
:
https://github.com/RobotLocomotion/drake/issues/14827