docs: updated server-configuration.md on GirhubApp slug (#3728)

Added information how to obtain `--gh-app-slug` value form installed app

runatlantis.io/docs/server-configuration.md

@@ -524,7 +524,7 @@ This is useful when you have many projects and want to keep the pull request cle
   # or
   ATLANTIS_GH_APP_SLUG="myappslug"
   ```
-  A slugged version of GitHub app name shown in pull requests comments, etc (not `Atlantis App` but something like `atlantis-app`). Atlantis uses the value of this parameter to identify the comments it has left on GitHub pull requests. This is used for functions such as `--hide-prev-plan-comments`.
+  A slugged version of GitHub app name shown in pull requests comments, etc (not `Atlantis App` but something like `atlantis-app`). Atlantis uses the value of this parameter to identify the comments it has left on GitHub pull requests. This is used for functions such as `--hide-prev-plan-comments`. You need to obtain this value from your GitHub app, one way is to go to your App settings and open "Public page" from the left sidebar. Your `--gh-app-slug` value will be the last part of the URL, e.g `https://github.com/apps/<slug>`.
 
 ### `--gh-app-key-file`
   ```bash