Fix server echo example

Author: h2zero

examples/NimBLE_Stream_Echo/NimBLE_Stream_Echo.ino

@@ -22,7 +22,8 @@ void setup() {
 
     // Initialize BLE
     NimBLEDevice::init("BLE-Echo");
-    NimBLEDevice::createServer();
+    auto pServer = NimBLEDevice::createServer();
+    pServer->advertiseOnDisconnect(true); // Keep advertising after clients disconnect
 
     /**
      * Initialize the stream server with: