[SPARK-50978][PYTHON][SS] Fix naming convention for mapStateClient

### What changes were proposed in this pull request?

Fix naming convention for mapStateClient. The parameter name should start with lowercase as `mapStateClient` while it's currently `MapStateClient`.

### Why are the changes needed?

Didn't follow best practice for parameter naming.

### Does this PR introduce _any_ user-facing change?

Yes.

### How was this patch tested?

Existing unit test.

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #49656 from bogao007/mapStateClient-fix.

Authored-by: bogao007 <[email protected]>
Signed-off-by: Jungtaek Lim <[email protected]>

Author: bogao007

python/pyspark/sql/streaming/stateful_processor.py

@@ -173,10 +173,10 @@ class MapState:
 
     def __init__(
         self,
-        MapStateClient: MapStateClient,
+        mapStateClient: MapStateClient,
         stateName: str,
     ) -> None:
-        self._mapStateClient = MapStateClient
+        self._mapStateClient = mapStateClient
         self._stateName = stateName
 
     def exists(self) -> bool: