Typo in startup.c comment

[ayoung19]

I think it's supposed to say 32.768kHz here: https://github.com/arduino/ArduinoCore-samd/blob/89833081ef18aea430b8943ab653dbbd510d43ce/cores/arduino/startup.c#L30

I know you can figure this out from the data sheet but since this modifies the chip's defaults it should be a little more clear, it confused me at first and might save other people time.