fixes config.js

comments in config.js must begin with two slashes

docs/configuring-playbook-jitsi.md

@@ -99,7 +99,7 @@ matrix_jitsi_web_custom_config_extension: |
 
   config.disableAudioLevels = true;
 
-  # Limit the number of video feeds forwarded to each client
+  // Limit the number of video feeds forwarded to each client
   config.channelLastN = 4;
 
 matrix_jitsi_web_config_resolution_width_ideal_and_max: 480