image save has incorrect size for R503 sensor

[jerryneedell]

The examples are configured for an image of 256x288 pixels when downloading the fingerprint image.
https://github.com/adafruit/Adafruit_CircuitPython_Fingerprint/blob/master/examples/fingerprint_r503_rpi.py#L163

the R503 sensor only has a 192x192 image so the code should be modified.
I'll be happy to make this change when can get to it but if anyone else wants to try it first, please go ahead.