Typo in startup.c comment

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.

Open cores/arduino/startup.c at line 30 and compare the comment with the SAMD21 datasheet or the linked source context. Update the frequency wording if confirmed, then review the diff to ensure the change is limited to the comment.

Written by the indexing model from the issue text.