fix: typo in description of markup_currency

Co-authored-by: João Bernardo <[email protected]>

Author: Norberto Lopes

duffel_api/api/links/sessions.py

@@ -162,7 +162,7 @@ def markup_amount(self, markup_amount: str):
 
     def markup_currency(self, markup_currency: str):
         """The currency of the markup_amount. It should always match the settlement
-        currency of the organisation. This field is required is markup_amount is provided.
+        currency of the organisation. This field is required if markup_amount is provided.
 
         Example: "GBP"