Fix period entry in font table

[caternuson]

For #53. This was fixed by #37, but that PR has merge conflicts now. This PR is same thing.

#include "Adafruit_LEDBackpack.h"

Adafruit_AlphaNum4 alpha4;

void setup() {
  alpha4.begin();

  alpha4.writeDigitAscii(0, '.');
  alpha4.writeDigitAscii(1, '.');
  alpha4.writeDigitAscii(2, '.');
  alpha4.writeDigitAscii(3, '.');

  alpha4.writeDisplay();
}

void loop() {
}

BEFORE

AFTER