Fix async context in onchain client examples

Updated the Ethers.js and Viem examples in `src/pages/build/onchain-clients.mdx` to wrap `await` calls in an explicit `async` function, since both examples previously used `await` without providing an async execution context.

Author: t0mw4lker

src/pages/build/onchain-clients.mdx

@@ -25,16 +25,17 @@ To connect to Ink by instantiating a new `ethers.js` `JsonRpcProvider` object wi
 
    const rpcUrl = 'https://rpc-gel-sepolia.inkonchain.com';
    const provider = new ethers.JsonRpcProvider(rpcUrl, 763373);
+
+   async function main() {
+     const blockNumber = await provider.getBlockNumber();
+     console.log(blockNumber);
+   }
+   
+   main().catch(console.error);
    ```
 
 3. **Using the Provider**: You can now use the `provider` to interact with Ink's testnet. For example, you can fetch the current block number or interact with smart contracts deployed on the testnet.
 
-   ```javascript
-   // previous code
-   const blockNumber = await provider.getBlockNumber();
-   console.log(blockNumber);
-   ```
-
 ### Example: Fetching the Current Block Number
 
 The following code snippet demonstrates how to fetch and print the current block number from Ink's testnet:
@@ -64,12 +65,13 @@ Viem documentation: [https://viem.sh/docs/introduction](https://viem.sh/docs/int
      chain: inkSepolia,
      transport: http(),
    })
+
+   async function main() {
+     const blockNumber = await client.getBlockNumber();
+     console.log(blockNumber);
+   }
+   
+   main().catch(console.error);
    ```
 
 3. **Consuming Actions**: You can now interact with Ink's chain! Here we are getting the current block number.
-
-   ```javascript
-   // previous code
-   const blockNumber = await client.getBlockNumber();
-   console.log(blockNumber);
-   ```