Update rxnutils/routes/base.py

Co-authored-by: Christos Kannas <chriskannas@gmail.com>

rxnutils/routes/base.py

@@ -86,7 +86,7 @@ def assign_atom_mapping(
     ) -> None:
         """
         Assign atom-mapping to each reaction in the route and
-        ensure that is is consistent from root compound and throughout
+        ensure that it is consistent from root compound and throughout
         the route.
 
         It will use NameRxn to assign classification and possiblty atom-mapping,