docs: fix group name

Built-in spark commands use group names with the first letter upper case.

Author: kenjis

user_guide_src/source/cli/cli_commands.rst

@@ -49,7 +49,7 @@ should contain the following code:
 
 .. literalinclude:: cli_commands/002.php
 
-If you run the **list** command, you will see the new command listed under its own ``demo`` group. If you take
+If you run the **list** command, you will see the new command listed under its own ``Demo`` group. If you take
 a close look, you should see how this works fairly easily. The ``$group`` property simply tells it how to organize
 this command with all of the other commands that exist, telling it what heading to list it under.
 

user_guide_src/source/cli/cli_commands/002.php

@@ -7,7 +7,7 @@
 
 class AppInfo extends BaseCommand
 {
-    protected $group       = 'demo';
+    protected $group       = 'Demo';
     protected $name        = 'app:info';
     protected $description = 'Displays basic application information.';