Update instructions in documentation for OAuth examples

[zimeg]

Summary

This PR makes a few changes to the OAuth examples in documentation to remove some confusion:

• Specifies that the /slack/oauth is added to the server URL for *_PATH environment variables to avoid unknowingly creating paths like /slack/oauth/slack/oauth/start.
• Updates the map entries when creating a server to make the map mutable to avoid a compilation error.

Category

• [x] bolt (Bolt for Java)
• [ ] bolt-{sub modules} (Bolt for Java - optional modules)
• [ ] slack-api-client (Slack API Clients)
• [ ] slack-api-model (Slack API Data Models)
• [ ] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
• [ ] slack-app-backend (The primitive layer of Bolt for Java)
• [x] docs (Documentation)

Requirements

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (60fc261) 74.31% compared to head (2f2a08b) 74.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1262 +/- ## ========================================= Coverage 74.31% 74.31% Complexity 4120 4120 ========================================= Files 443 443 Lines 13078 13078 Branches 1322 1322 ========================================= Hits 9719 9719 Misses 2587 2587 Partials 772 772 ```

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