typo fix in order_and_orient argument help text (#733)

assembly.py

@@ -559,7 +559,7 @@ def parser_order_and_orient(parser=argparse.ArgumentParser()):
     parser.add_argument(
         'inReference', nargs='+',
         help=('Reference genome for ordering, orienting, and merging contigs, FASTA format.  Multiple filenames may be listed, each '
-              'containing one reference genome. Alternattely, multiple references may be given by specifying a single filename, '
+              'containing one reference genome. Alternatively, multiple references may be given by specifying a single filename, '
               'and giving the number of reference segments with the nGenomeSegments parameter.  If multiple references are given, '
               'they must all contain the same number of segments listed in the same order.')
     )