RFR java.time.Duration spec correction (8031103)

[roger riggs]

Please review this minor specification correction to the 
java.time.Duration.toDays() and
toHours() methods.  Only the javadoc is corrected, no code or tests are 
affected.

Webrev:
http://cr.openjdk.java.net/~rriggs/webrev-duration-javadoc-8031103/
Thanks, Roger

JDK-8031103 <https://bugs.openjdk.java.net/browse/JDK-8031103> 
java.time.Duration has wrong Javadoc Comments in toDays() and toHours()