Update `rebase_onto` demo test to match new the rebase menu title (#3636)

- **PR Description**

This PR updates the `rebase_onto` demo integration test, which is
currently failing on master due to [this
change](https://github.com/jesseduffield/lazygit/pull/3615/files#diff-3eb5426752dae525c92f1ecce2f7215d15bfdec91bd459e08f78574876583910R1259).
Our CI does not throw the issue, but it's still annoying to cope with it
for local development.

@jesseduffield @stefanhaller, I'm unaware of a particular workflow for
updating demo tests, but let me know if this needs some tweaks.

- **Please check if the PR fulfills these requirements**

* [ ] Cheatsheets are up-to-date (run `go generate ./...`)
* [ ] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [ ] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [ ] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [ ] Docs have been updated if necessary
* [ ] You've read through your own file changes for silly mistakes etc

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