Fix autoscaler format string for memory (#5542)

* add format string

* fix cast

python/ray/services.py

@@ -1301,7 +1301,7 @@ def _start_plasma_store(plasma_store_memory,
                         "plasma_directory argument must be provided.")
 
     if not isinstance(plasma_store_memory, int):
-        raise Exception("plasma_store_memory should be an integer.")
+        plasma_store_memory = int(plasma_store_memory)
 
     command = [
         PLASMA_STORE_EXECUTABLE, "-s", socket_name, "-m",