Documentation: ABI: testing: Fix "upto" typo in rtc-cdev

The word "upto" is a common typo for "up to". Correct this.

Signed-off-by: Akiyoshi Kurita <weibu@redadmin.org>
Link: https://patch.msgid.link/20251112113759.2953758-1-weibu@redadmin.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>