message: Copy: Fix doctest.
diff: @@ -2989,11 +2989,11 @@ class Copy(Builder):
  >>> with open(str(source.path()), 'w') as f:
  ... print('Content.', file = f)
  >>> builder = Copy(source, Path('/tmp/.drake.Copy.dest'))
  - >>> target = builder.target()
  + >>> target = builder.target
  >>> target
  /tmp/.drake.Copy.dest
  >>> target.path().remove()
  - >>> builder.target().build()
  + >>> builder.target.build()
  Copy /tmp/.drake.Copy.dest
  >>> with open(str(target.path()), 'r') as f:
  ... content = f.read()
  