Replace rspec with sus in Getting Started guide

[mjq]

The test that comes with utopia:site:create now uses sus instead of rspec. Updated the Getting Started guide to match.

The complete output of sus on a fresh site was actually

1 samples: 1x 200. 3703.7 requests per second. S/D: 0.000µs.
1 passed out of 1 total (2 assertions)
🏁 Finished in 247.4ms; 8.085 assertions per second.
😭 You should write more tests and assertions!
🐢 Slow tests:
        247.3ms: describe website it should be responsive test/website.rb:16

but I thought the first three lines got the point across - happy to include the rest of the message if you'd prefer. Thanks!

Types of Changes

• Documentation.

Contribution

• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[samuel-williams-shopify]

Thanks for your contribution!