Motor 4 functions should use OCR5A, not OCR5B

Pins Reference [see link](http://astro.neutral.org/arduino/arduino-pwm-pins-frequency.shtml)
Addresses issue #6
| Arduino Pin | Register |
|-------------|----------|
| 2           | OCR3B    |
| 3           | OCR3C    |
| 4           | OCR4C    |
| 5           | OCR3A    |
| 6           | OCR4A    |
| 7           | OCR4B    |
| 8           | OCR4C    |
| 9           | OCR2B    |
| 10          | OCR2A    |
| 11          | OCR1A    |
| 12          | OCR1B    |
| 13          | OCR0A    |
| 44          | OCR5C    |
| 45          | OCR5B    |
| 46          | OCR5A    |

DualVNH5019MotorShieldMod3.cpp

@@ -440,7 +440,7 @@ void DualVNH5019MotorShieldMod3::setM4Speed(int speed)
   #ifdef DUALVNH5019MOTORSHIELD_TIMER5_AVAILABLE
     if (_PWM3 == _PWM3_TIMER5_PIN && _PWM4 == _PWM4_TIMER5_PIN)
     {
-      OCR5B = speed;
+      OCR5A = speed;
     }
     else
     {
@@ -592,7 +592,7 @@ void DualVNH5019MotorShieldMod3::setM4Brake(int brake)
   #ifdef DUALVNH5019MOTORSHIELD_TIMER5_AVAILABLE
     if (_PWM3 == _PWM3_TIMER5_PIN && _PWM4 == _PWM4_TIMER5_PIN)
     {
-      OCR5B = brake;
+      OCR5A = brake;
     }
     else
     {