Adjust deprecation message on Duration.weeks.

Apparently, some folks found it confusing, because it just so happens
that get!"weeks"() was identical to total!"weeks", since weeks were the
largest units. However, it does seem to highlight how folks have
misunderstood what the individual getters do (which is why they're being
deprecated).
Merge pull request #837 from jmdavis/message

Adjust deprecation message on Duration.weeks.