fix minor typo (#519)

Author: hirosassa

gokart/in_memory/data.py

@@ -11,5 +11,5 @@ class InMemoryData:
     last_modification_time: datetime
 
     @classmethod
-    def create_data(self, value: Any) -> InMemoryData:
+    def create_data(cls, value: Any) -> InMemoryData:
         return InMemoryData(value=value, last_modification_time=datetime.now())