[examples] Invert right side of drive subsystems (#3437)

The right motors of a DifferentialDrive are no longer automatically
inverted (#3340) so it needs to be done explicitly.

wpilibcExamples/src/main/cpp/examples/SimpleDifferentialDriveSimulation/include/Drivetrain.h

@@ -31,6 +31,12 @@ class Drivetrain {
  public:
   Drivetrain() {
     m_gyro.Reset();
+
+    // We need to invert one side of the drivetrain so that positive voltages
+    // result in both sides moving forward. Depending on how your robot's
+    // gearbox is constructed, you might have to invert the left side instead.
+    m_rightGroup.SetInverted(true);
+
     // Set the distance per pulse for the drive encoders. We can simply use the
     // distance traveled for one rotation of the wheel divided by the encoder
     // resolution.