docs(gh-pages): improve separate repository instructions (#35118)

* docs(gh-pages): improve separate repository instructions

* Adjust sentence slightly

Co-authored-by: Ty Hopp <tyhopp@users.noreply.github.com>

docs/docs/how-to/previews-deploys-hosting/how-gatsby-works-with-github-pages.md

@@ -69,7 +69,7 @@ For a repository named like `username.github.io`, you don't need to specify `pat
 >      `git checkout -b source main`
 >   2. Change the default branch in your repository settings ("Branches" menu item) from `main` to `source`
 > - **Note**: GitHub Pages lets you use any branch for deployment, see [this docs page](https://docs.github.com/en/pages/getting-started-with-github-pages/configuring-a-publishing-source-for-your-github-pages-site#choosing-a-publishing-source) on how to do this. This means you do not necessarily have to change your default branch.
-> - Have a separate repository for your source code (so `username.github.io` is used only for deployment and not really for tracking your source code)
+> - Have a separate repository for your source code (so `username.github.io` is used only for deployment and not really for tracking your source code). If you go down this route, you will need to add an extra option for `--repo <repo>` (works for https and git urls) in the gh-pages command below.
 
 ```json:title=package.json
 {