[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/DifferentialDrivePoseEstimator/include/Drivetrain.h

@@ -24,6 +24,11 @@
 class Drivetrain {
  public:
   Drivetrain() {
+    // 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);
+
     m_gyro.Reset();
     // 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