fix the order of args in the custom_seqinfo doc

Author: yarikoptic

docs/heuristics.rst

@@ -109,7 +109,7 @@ or::
         return seqinfos  # ordered dict containing seqinfo objects: list of DICOMs
 
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-``custom_seqinfo(series_files, wrapper)``
+``custom_seqinfo(wrapper, series_files)``
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 If present this function will be called on each group of dicoms with
 a sample nibabel dicom wrapper to extract additional information