Import SessionContext in the SessionConfig constructor doctest

The example used SessionContext but only imported SessionConfig, so it
passed under --doctest-modules yet failed when copy-pasted.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: timsaucer

python/datafusion/context.py

@@ -180,7 +180,7 @@ def __init__(self, config_options: dict[str, str] | None = None) -> None:
 
         Example usage:
 
-        >>> from datafusion import SessionConfig
+        >>> from datafusion import SessionConfig, SessionContext
         >>> ctx = SessionContext(SessionConfig())
         >>> config = SessionConfig(
         ...     config_options={"datafusion.execution.batch_size": "1024"}