fix the order of args in the custom_seqinfo doc

Author: yarikoptic

docs/heuristics.rst

@@ -120,7 +120,7 @@ or::
         return seqinfos  # ordered dict containing seqinfo objects: list of DICOMs
 
 ---------------------------------------------------------------
-``custom_seqinfo(series_files, wrapper)``
+``custom_seqinfo(wrapper, series_files)``
 ---------------------------------------------------------------
 If present this function will be called on each group of dicoms with
 a sample nibabel dicom wrapper to extract additional information