Vouchers: Fix field label inconsistency (Z#23222887) (#5902)

The field Voucher.price_mode is sometimes called "Price mode" and
sometimes "Price effect" in the UI, which is inconsistent. I think
"price effect" is a little clearer, but I don't really care as long as
it is consistent.

src/pretix/base/models/vouchers.py

@@ -239,7 +239,7 @@ class Voucher(LoggedModel):
         )
     )
     price_mode = models.CharField(
-        verbose_name=_("Price mode"),
+        verbose_name=_("Price effect"),
         max_length=100,
         choices=PRICE_MODES,
         default='none'