bitcoin-paper: Ensure Korean filename matches ISO standard

_templates/bitcoin-paper.html

@@ -202,7 +202,7 @@ id: bitcoin-paper
 
         <li class="card bitcoin-paper-card">
           <a class="language-link"
-            href="/files/bitcoin-paper/bitcoin_kr.pdf">한국어</a>
+            href="/files/bitcoin-paper/bitcoin_ko.pdf">한국어</a>
           <div>
             <span>{% translate translated_by %}</span>
             <a href="https://github.com/imc84">Mincheol Im</a>