Update hello_sleep.c

Comment should generally match the action.

sleep/hello_sleep/hello_sleep.c

@@ -29,7 +29,7 @@ static void rtc_sleep(void) {
             .sec   = 00
     };
 
-    // Alarm 30 seconds later
+    // Alarm 10 seconds later
     datetime_t t_alarm = {
             .year  = 2020,
             .month = 06,
@@ -74,4 +74,4 @@ int main() {
     }
 
     return 0;
-}
+}