fix changelog

Author: brozorec

content/stellar-contracts/changelog.mdx

@@ -23,6 +23,7 @@ title: Changelog
 * Bound metadata entries in rwa in https://github.com/OpenZeppelin/stellar-contracts/pull/511
 * Allowance bug fix in https://github.com/OpenZeppelin/stellar-contracts/pull/513
 * Vault consistency fix in https://github.com/OpenZeppelin/stellar-contracts/pull/523
+* Contract trait, deprecate `default_impl` macro in https://github.com/OpenZeppelin/stellar-contracts/pull/525
 
 [Changes][v0.6.0]