chore: added solidity highlights for code snippet (#1365)

Co-authored-by: Idris Olubisi <[email protected]>

Author: benjamin852

src/content/docs/dev/send-tokens/interchain-tokens/register-existing-token.mdx

@@ -89,7 +89,7 @@ When tokens are sent to this contract, the `executeWithInterchainToken` function
 
 Example implementation:
 
-```
+```solidity
 // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;