Clarify that one needs to explicitly pass custom `persist_dir` to `storage_context.persist` (#9694)

docs/getting_started/starter_example.md

@@ -95,15 +95,16 @@ from llama_index import (
 )
 
 # check if storage already exists
-if not os.path.exists("./storage"):
+PERSIST_DIR = "./storage"
+if not os.path.exists(PERSIST_DIR):
     # load the documents and create the index
     documents = SimpleDirectoryReader("data").load_data()
     index = VectorStoreIndex.from_documents(documents)
     # store it for later
-    index.storage_context.persist()
+    index.storage_context.persist(persist_dir=PERSIST_DIR)
 else:
     # load the existing index
-    storage_context = StorageContext.from_defaults(persist_dir="./storage")
+    storage_context = StorageContext.from_defaults(persist_dir=PERSIST_DIR)
     index = load_index_from_storage(storage_context)
 
 # either way we can now query the index