Minor note on AlFworld system prompt

[qishenghu]

Hi Devs,

Not a big issue, but I noticed that the system prompt template for AlFworld includes the sentence "buy the item by yourself." I think this might have come from the Webshop task.

Just wanted to flag it in case it was unintentional.

[garyzhang99]

Thanks! It was unintentional; we will fix this.