Update page.mdx (#6270)

Update "extension" to "extensions"

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## PR-Codex overview
This PR updates the import path for the `balanceOf` function from the `thirdweb/extension/erc1155` to `thirdweb/extensions/erc1155`, ensuring that the correct module is referenced for checking wallet balances.

### Detailed summary
- Changed the import statement for `balanceOf` from `thirdweb/extension/erc1155` to `thirdweb/extensions/erc1155`.
- Added a newline at the end of the file.

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Author: warengonzaga

apps/portal/src/app/react/v5/migrate/contracts/page.mdx

@@ -33,7 +33,7 @@ Check out the list of over 100 prebuilt extensions [here](/typescript/v5/extensi
 
 Example: Import an ERC1155 "read" extension, for checking the balance of a wallet
 ```tsx
-import { balanceOf } from "thirdweb/extension/erc1155";
+import { balanceOf } from "thirdweb/extensions/erc1155";
 
 const transaction = balanceOf({
   contract,
@@ -169,4 +169,4 @@ function App() {
     Claim
   </TransactionButton>
 }
-```
+```