change readme

Author: zaraheath

README.md

@@ -62,7 +62,7 @@ amount_code=2,
 document_number=0,
 origin_branch_code=123,
 own_item=16,
-reference_1=0,
+reference_1=nil,
 reference_2=nil,
 shared_item=4,
 transaction_date=Tue, 04 Feb 2016,