fix: Fix f-string bugs in charging log message (#817)

## Summary

- Fixed `${subject}` → `{subject}` in the charging log message — the `$`
prefix caused the variable to be printed literally instead of
interpolated.
- Added missing `f` prefix to the continuation string so
`{charged_count}` gets properly interpolated.

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)