[SDK] Fix tsdocs for isERC20 (#5609)

## Problem solved

Short description of the bug fixed or feature added

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## PR-Codex overview
This PR updates the `isERC20` function in the `isERC20.ts` file to retrieve the contract's ABI and extract function selectors before checking if the contract is an ERC20.

### Detailed summary
- Removed direct usage of `isERC20` with a contract.
- Added import of `resolveContractAbi` from `thirdweb/contract`.
- Introduced a new step to resolve the contract's ABI.
- Extracted function selectors from the ABI.
- Updated `isERC20` call to use the extracted selectors.

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Author: kien-ngo

packages/thirdweb/src/extensions/erc20/read/isERC20.ts

@@ -16,7 +16,14 @@ import { isTransferFromSupported } from "../__generated__/IERC20/write/transferF
  * @example
  * ```ts
  * import { isERC20 } from "thirdweb/extensions/erc20";
- * const result = await isERC20({ contract });
+ * import { resolveContractAbi } from "thirdweb/contract";
+ *
+ * const abi = await resolveContractAbi(contract);
+ * const selectors = abi
+ *   .filter((f) => f.type === "function")
+ *   .map((f) => toFunctionSelector(f));
+ *
+ * const result = await isERC20(selectors);
  * ```
  */
 export function isERC20(availableSelectors: string[]) {