Tweaking getting started docs to reflect changes in app setup UI on the application configuration pages

[filmaj]

Summary

It looks like the various app configuration pages on api.slack.com/apps have changed their UI since the Getting Started docs in this project were written. This PR has a few tiny tweaks to the getting started instructions to reflect the changes in the UI.

Also changed the example tutorial code so that the "Message Posted" console.log only runs if no error is raised. Otherwise, in case you hit an error while running the example tutorial code (if you're silly like I was and forgot to invite my bot to the general channel), you could see something like this:

➜ node tutorial.js
Getting started with Node Slack SDK
Error: An API error occurred: not_in_channel
    at Object.platformErrorFromResult (/Users/fmaj/src/slapp-node-web-api/node_modules/@slack/web-api/dist/errors.js:51:33)
    at WebClient.apiCall (/Users/fmaj/src/slapp-node-web-api/node_modules/@slack/web-api/dist/WebClient.js:158:28)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at async /Users/fmaj/src/slapp-node-web-api/tutorial.js:11:5 {
  code: 'slack_webapi_platform_error',
  data: {
    ok: false,
    error: 'not_in_channel',
    response_metadata: { scopes: [Array], acceptedScopes: [Array] }
  }
}
Message posted!

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.