Fix a small typo in `SystemClock` class docs.

 *     sleep (CPU off, display dark, device waiting for external input),

 *     but is not affected by clock scaling, idle, or other power saving

 *     mechanisms.  This is the basis for most interval timing

 *     such as {@link Thread#sleep(long) Thread.sleep(millls)},

 *     such as {@link Thread#sleep(long) Thread.sleep(millis)},

 *     {@link Object#wait(long) Object.wait(millis)}, and

 *     {@link System#nanoTime System.nanoTime()}.  This clock is guaranteed

 *     to be monotonic, and is suitable for interval timing when the