Incorrect handling of the "-" character on Seg7x4 displays

The problem is located on the following line:

https://github.com/adafruit/Adafruit_CircuitPython_HT16K33/blob/94453bf8fdcd0578a7957fc401ccc261ed46b68c/adafruit_ht16k33/segments.py#L464

Offset 16 for the "-" character is no longer valid. The closest valid offset is 20, but I don't know if this is a correct solution.
