AK: Append correct number of port characters when serializing a URL

Instead of formatting a port string, it put bytes from stack, using the
port number as a length (so for port 8000 it appended 8000 bytes).

AK/URL.cpp

@@ -347,7 +347,7 @@ String URL::serialize_origin() const
     builder.append("://"sv);
     builder.append(m_host);
     if (m_port.has_value())
-        builder.append(":{}", *m_port);
+        builder.appendff(":{}", *m_port);
     return builder.build();
 }