Add extra select-auth step

[wongjas]

Type of change (place an x in the [ ] that applies)

• New sample
• New feature
• Bug fix
• Documentation

Summary

As of v2.3.0, after using the slack external-auth add command, you need to use the slack external-auth select-auth command if credential_source: "DEVELOPER". Adding this into the instructions since the instructions error out without this at the moment.

Requirements (place an x in each [ ] that applies)

• I’ve checked my submission against the Samples Checklist to ensure it complies with all standards
• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct

[zimeg]

Really appreciate this addition, thank you! 🙌

There's a formatting issue from line length that was causing the CI to fail, but running a quick deno fmt can fix this.

I also left a note that adds some detail for this command, but your call on this change!

Approving now since this change is good, but I'm also wondering if we might update this sample to use end user authentication instead? I'll check on this, not a blocker to this PR!