mod_pubsub_github/README: Specify language of code block for pretty syntax highlighting

--- a/mod_pubsub_github/README.markdown	Sun Mar 31 19:05:22 2019 +0200
+++ b/mod_pubsub_github/README.markdown	Sun Mar 31 19:05:48 2019 +0200
@@ -17,9 +17,11 @@
 
 Load the module on a pubsub component:
 
-    Component "pubsub.example.com" "pubsub"
-        modules_enabled = { "pubsub_github" }
-        github_secret = "NP7bZooYSLKze96TQMpFW5ov"
+``` {.lua}
+Component "pubsub.example.com" "pubsub"
+    modules_enabled = { "pubsub_github" }
+    github_secret = "NP7bZooYSLKze96TQMpFW5ov"
+```
 
 The URL for Github to post to would be either: