DOC: update docstring to reflect new tolerance

Docstring incorrect in using S.shape rather than the shape of the
(matrix part of the) affine.

Author: matthew-brett

nibabel/orientations.py

@@ -37,7 +37,7 @@ def io_orientation(affine, tol=None):
        threshold below which SVD values of the affine are considered zero. If
        `tol` is None, and ``S`` is an array with singular values for `affine`,
        and ``eps`` is the epsilon value for datatype of ``S``, then `tol` set to
-       ``S.max() * max(S.shape) * eps``.
+       ``S.max() * max((q, p)) * eps``
 
     Returns
     -------