docs: fix System.CommandLine tab-completion example to use RootCommand

[abhiramch018]

Summary

Fixed the tab-completion example in how-to-enable-tab-completion.md by updating the sample code to use RootCommand instead of Command.

Changes

• Wrapped the command in a RootCommand.
• Updated the invocation to use rootCommand.Parse(args).InvokeAsync().
• Verified that tab-completion and help now work correctly.

Why

Fixes #48478 — the original code used .Parse(args).Invoke() on a Command, which prevented tab-completion and help from working as intended.

Summary

Describe your changes here.

Fixes #Issue_Number (if available)