Fix "Push to GitHub" section

If the user doesn't have `push.autoSetupRemote` set to `true` in their
`.gitconfig`, then this step will error with "The current branch
<branch> has no upstream branch", which is confusing.

Thanks for reporting this, @rose-higgins.

docs/getting-started/how-to/add-github-codespaces-to-your-project/index.md

@@ -95,7 +95,7 @@ git commit --message="Add GitHub Codespaces"
 Push to GitHub:
 
 ```sh
-git push
+git push --set-upstream
 ```
 
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