Use port from variable instead of hard coding

### Changes
1. Edited error description to add port's value from variable instead of hard coding. Also removed redundant `//` between localhost and port.

src/platforms/electron/startSandbox.js

@@ -31,7 +31,7 @@ export default async function startSandbox() {
     .on('error', err => {
       if (err.code === 'EADDRINUSE') {
         console.log(
-          'Server already listening at localhost://5278: This is probably another LBRY app running. If not, games in the app will not work.'
+          `Server already listening at localhost:${port}. This is probably another LBRY app running. If not, games in the app will not work.`
         );
       }
     });