Fix hardcoded WG_PORT in js

Had already added `$WG_PORT` variable to readme, but negated to switch it to a variable within the .js

src/lib/WireGuard.js

@@ -95,7 +95,7 @@ module.exports = class WireGuard {
 [Interface]
 PrivateKey = ${config.server.privateKey}
 Address = ${config.server.address}/24
-ListenPort = 51820
+ListenPort = ${WG_PORT}
 PreUp = ${WG_PRE_UP}
 PostUp = ${WG_POST_UP}
 PreDown = ${WG_PRE_DOWN}