Documentation: rtc: Correct location of rtctest.c

The useful little rtctest program moved location a while back.

Fixes: a12ab9e125f1 ("selftests: move RTC tests to rtc subfolder")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed