fix: fix pollstatus issue

[wyangsun]

• fix the issue where the pollstatus was not working correctly
• fix the issue where the vectorstoreid was not being passed correctly in example

Error logs:

go run main.go -- test0.txt test1.txt
File to upload: test0.txt
File to upload: test1.txt
Creating a new vector store and uploading files
panic: vector store file batch poll: received GET "https://api.openai.com/v1/vector_stores/vsfb_***/file_batches/vs_***": 400 Bad Request {
            "message": "Invalid 'batch_id': 'vs_686329ef3c6081918a21b7a9fa904dd4'. Expected an ID that begins with 'vsfb_'.",
            "type": "invalid_request_error",
            "param": "batch_id",
            "code": "invalid_value"
          }

goroutine 1 [running]:
main.main()