docs(key-wallet): fix broken receive-address example in IMPLEMENTATION_SUMMARY

The example called `get_next_receive_address()`, which is not a method on
this struct. Replace with the real `next_receive_address(...)` signature so
the snippet compiles.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: QuantumExplorer

key-wallet/IMPLEMENTATION_SUMMARY.md

@@ -183,7 +183,7 @@ let account = wallet.create_account(
 let mut managed = ManagedCoreFundsAccount::from_account(&account);
 
 // Get a receive address
-let address = managed.get_next_receive_address()?;
+let address = managed.next_receive_address(Some(&account.account_xpub), true)?;
 ```
 
 ## Dependencies