specs: fix a garbage-in, garbage-out example.

Close #44.
* specs/tree_spec.yaml (tostring): The assumption that tostring
should recurse by itself, or that the Tree constructor should
massage subtables on instantiation were both flawed.  Fix the
input to be properly nested tree, and `tostring` will indeed
output a properly nested tree.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>