DYN-7082 Update DSCore.List.IndexOf node help doc (DynamoDS#15453)

doc/distrib/NodeHelpFiles/DSCore.List.IndexOf.md

@@ -1,5 +1,5 @@
 ## In Depth
-`List.IndexOf` returns the index location of a given element in a list. If the given element isn’t found in the list or its data type does not match the list element (for example, the input is a double and the list element is a string), `List.IndexOf` returns -1. 
+`List.IndexOf` returns the index location of a given element in a list. If the given element isn’t found in the list or its data type does not match the list element (for example, the input is a double and the list element is a string), `List.IndexOf` returns null. 
 
 Note that this node returns only the first index, even if the element appears multiple times on the list. To return multiple indices, use `List.AllIndicesOf` instead.
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