RedditService service fails with 403 Blocked error

There is an easy fix for this, we need to add a 'User-Agent' header, something like below:
```csharp
public interface IRedditService
{
///
/// Get a list of posts from a given subreddit
///
/// The subreddit name.
[Headers("User-Agent: Awesome CommunityToolkit.Mvvm Sample App")]
[Get("/r/{subreddit}/.json")]
Task GetSubredditPostsAsync(string subreddit);
}
```
Now the sample in the 'The Final Result!' will work correctly.