readme update (#292)

Co-authored-by: Djoyke Reijans <djoyke.reijanscruden@adyen.com>

README.md

@@ -155,7 +155,7 @@ adyen.management.account_company_level_api.get_companies(query_parameters=query_
 Define a dictionary containing the headers you want to include in your request.
 
 ~~~~ python    
-    header_parameters = {
+    headers = {
         "Var1": "Var2",
         "Var2": "Var1"
     }