So, I'm guessing your using the exact same code as the one in the tutorial?
If so, try making these changes
int threshold=10;//the larger this number, the more likely your robot will drive straight
LED_on();
//detects more light on left side of robot
if(sensor_left > sensor_right && (sensor_left - sensor_right) > threshold)
{//go left
servo_left(30);
servo_right(30);
}
//detects more light on right side of robot
else if(sensor_right > sensor_left && (sensor_right - sensor_left) > threshold)
{//go right
servo_left(20);
servo_right(20);
}
//light is about equal on both sides
else
{//go straight
servo_left(20);
servo_right(30);
}