Rename "Single Wallet" to "Unified Wallet"

I've seen the term unified wallet more recently, and it seems to be more descriptive. If others agree, let's change the LDK home page.

Author: moneyball

docs/index.md

@@ -44,7 +44,7 @@ Use a local full node, an SPV node, or your own Electrum server with API access
 </div>
 
 <div class="feature">
-<h3>Single Wallet</h3>
+<h3> Unified Wallet</h3>
 
 No need to create separate Bitcoin & Lightning wallets, forcing users to backup an additional recovery phrase. Instead, we let you define your own wallet and create one unified experience.