Update how-gatsby-works-with-github-pages.md (#38909)

Clarified that the deploy command should be run in the main branch for default setup.

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

@@ -60,7 +60,7 @@ Then add a `deploy` script to `package.json` in your repository's codebase:
 }
 ```
 
-When you run `npm run deploy` all contents of the `public` folder will be moved to your repository's `gh-pages` branch. Make sure that your repository's settings has the `gh-pages` branch set as the source to deploy from.
+When you run `npm run deploy` in your `main` branch all contents of the `public` folder will be moved to your repository's `gh-pages` branch. Make sure that your repository's settings has the `gh-pages` branch set as the source to deploy from.
 
 **Note**: To select `main` or `gh-pages` as your publishing source, you must have the branch present in your repository. If you don't have a `main` or `gh-pages` branch, you can create them and then return to source settings to change your publishing source.