Realtime Update Example

#### Is your feature request related to a problem? Please describe
Step 7 of the Hello World Tutorial shows users how Wave content can be updated in real time. This is focused on the code and makes some assumptions that the end user 1. knows what REPL is and 2. can extrapolate that this example is showing them how Wave can show real-time updates to all users.

#### Describe the solution you'd like
Add some more context before walking through the steps to really show what this section is doing and how important it is

Start with Step 7 of the Hello World Tutorial and review how it introduces the realtime update example. Add context explaining what REPL means and that the example demonstrates Wave updates appearing for all users; the tutorial should make this purpose clear before the existing steps.

Written by the indexing model from the issue text.