Update lrauv_control_demo.sdf

[alesof]

🦟 Bug fix

Related to #1646

Summary

To reproduce the error run the simulation in gazebo ionic. Output:

[error] The use of added mass through this plugin is deprecated and willbe removed in Gazebo J* as this formulation has instabilities. We recommend using the SDF `<fluid_added_mass>` tag based method[http://sdformat.org/spec?ver=1.11&elem=link#inertial_fluid_added_mass]To get rid of this warning we recommend setting`<disable_added_mass> to true.

By adding the suggested tag the warning is removed.