docs: add missing TokenStandard import to Bulk Transfers examples

[callumweb3]

Motivation

The Bulk Transfers examples in developerDocs/advanced-use-cases.md use TokenStandard.ERC721, TokenStandard.ERC1155, and TokenStandard.ERC20 without importing TokenStandard, making the code snippets non-functional when copied.

Evidence

Error without import: error TS2304: Cannot find name 'TokenStandard'

With import: success

Solution

Added by:

import { TokenStandard } from "@opensea/sdk";