Use another augmented assignment statement

:eyes: Some source code analysis tools can help to find opportunities for improving software components.
:thought_balloon: I propose to [increase the usage of augmented assignment statements](https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements "Augmented assignment statements") accordingly.

```diff
diff --git a/fluent/handler.py b/fluent/handler.py
index 7aefd8f..46fcce9 100644
--- a/fluent/handler.py
+++ b/fluent/handler.py
@@ -147,7 +147,7 @@ class FluentRecordFormatter(logging.Formatter, object):
if self.__style:
value = self.__style(value).format(record)
else:
- value = value % record.__dict__
+ value %= record.__dict__
except KeyError as exc:
value = None
if not self.fill_missing_fmt_key:
```

Start in fluent/handler.py at the FluentRecordFormatter formatting branch shown in the issue. Check the surrounding formatting behavior, make the augmented-assignment change while preserving the result, and verify the formatter still handles record dictionaries and missing keys as before.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.