Add secretKey to session SSE server.ts example

[diegoramoz]

Prevents error thrown when attempting to run server.ts without setting MPP_SECRET_KEY as an environment variable.

Error: Missing secret key. Set the MPP_SECRET_KEY environment variable or pass `secretKey` to Mppx.create().

The session WebSocket example uses the same pattern. I copied it from there. It makes it easier to get started testing.