correct sys.mass_a to sys.mass_a in FMU example

[maysam-gholampour]

correct sys.mass_a to sys.mass_a in FMU example

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

I only added _ for acceleration parameter inside the FMU importing example