Fix hypothetical Java example

The documentation had an example rule for a hypothetical compile of
a .java file that would compile down to a .class file, but the
example began with:

    rule '.java' => ...

Since the extension after 'rule' is supposed to be the target
extension rather than the source extension, it should begin with:

    rule '.class' => ...
Merge pull request #160 from mmorearty/patch-1

Fix hypothetical Java example