Include + in the refspec in sample commands for split push (2 GB limit)

[starrify]

Why:

The sample commands suggested in troubleshooting-the-2-gb-push-limit.md might potentially fail (rejected by GitHub) when the target branch contains non-linear history.

What's being changed (if available, include any code snippets, screenshots, or gifs):

This is considered to be a minor change as only two plus signs are added to the refspecs ($commit -> +$commit) in the sample commands.

As per the documentations on the refspec format of git push:

The special refspec `:` (or `+:` to allow non-fast-forward updates) directs Git to push "matching" branches: for every branch that exists on the local side, the remote side is updated if a branch of the same name already exists on the remote side. 

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• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

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[github-actions]

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This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

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Source	Preview	Production	What Changed
get-started/using-git/troubleshooting-the-2-gb-push-limit.md	fpt ghec	fpt ghec

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

@starrify Thanks so much for opening a PR! I'll get this triaged for review ✨

[github-actions]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert 👀

[nguyenalex836]

@starrify Thank you for your patience while our SME team reviewed! Your changes look wonderful ✨ I'll update the branch and get this merged once tests are passing 🍏

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues ⚡