fix: Fix f-string bugs in charging log message

Remove stray `$` before `{subject}` and add missing `f` prefix on the
second line so `{charged_count}` is actually interpolated instead of
rendered as a literal.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vdusek

src/apify/_charging.py

@@ -279,8 +279,8 @@ def calculate_chargeable() -> dict[str, int | None]:
         if charged_count < count:
             subject = 'instance' if count == 1 else 'instances'
             logger.info(
-                f"Charging {count} ${subject} of '{event_name}' event would exceed max_total_charge_usd "
-                '- only {charged_count} events were charged'
+                f"Charging {count} {subject} of '{event_name}' event would exceed max_total_charge_usd "
+                f'- only {charged_count} events were charged'
             )
 
         max_charge_count = self.calculate_max_event_charge_count_within_limit(event_name)