index: Add missing 'tree' entry to diff_to_tree docstring argspec

It's been missing since the original argspec was added with 5ed9eb4
(Add documentation for conflicts and fixup Index, 2014-07-10).

Maybe it's better to auto-generate these for the docs?  For example:

  >>> import inspect
  >>> import pygit2
  >>> argspec = inspect.getargspec(pygit2.index.Index.diff_to_tree)
  >>> inspect.formatargspec(args=argspec.args[1:], varargs=argspec.varargs,
  ...     varkw=argspec.keywords, defaults=argspec.defaults)
  '(tree, flags=0, context_lines=3, interhunk_lines=0)'

Where the [1:] is just to drop the 'self' entry.  I imagine there's
way to get Sphinx's automethod to do this sort of thing automatically,
but I don't know what that way is.