Fix a small bug in motor_writer.cc.

It caused it to disable the wrong PWM output for the drivetrain.

Change-Id: I0ff93c21dea018d57ed81e3cd1e4841e43b7e524
diff --git a/frc971/output/motor_writer.cc b/frc971/output/motor_writer.cc
index a81896f..19d431c 100644
--- a/frc971/output/motor_writer.cc
+++ b/frc971/output/motor_writer.cc
@@ -48,7 +48,7 @@
         SetSolenoid(8, drivetrain->right_high);
       } else {
         DisablePWMOutput(3);
-        DisablePWMOutput(8);
+        DisablePWMOutput(6);
         LOG_INTERVAL(drivetrain_old_);
       }
       drivetrain_old_.Print();