update index with error codes

Author: brozorec

content/stellar-contracts/index.mdx

@@ -60,6 +60,7 @@ Similarly, utilities and other modules have their own error codes:
   * Ownable: `21XX`
   * Role Transfer (internal common module for 2-step role transfer): `22XX`
 * Accounts: `3XXX`
+* Governance: `4XXX`
 
 ## Important Notes
 As a deliberate design choice, this library manages the TTL for temporary and persistent storage items.