Call on_change_party without assignment in SEPA mandate migration

The method updates the address in place and returns None. Assigning its
return value to the address raises an AssertionError.

Closes #14996

doc/migration.rst

@@ -108,7 +108,7 @@ After
          ])
 
      for mandate in mandates:
-         mandate.address = mandate.on_change_party()
+         mandate.on_change_party()
 
      Mandate.save(mandates)