Dispose IDisposable objects

We should call `Dispose` on objects that implement `IDisposable` in examples.

For example

```
HttpClient client = new HttpClient();
return client.GetStringAsync("https://docs.microsoft.com");
```

can be changed to

```
using (HttpClient client = new HttpClient())
{
    return client.GetStringAsync("https://docs.microsoft.com");
}
```