Add missing step in import_export.md

What does this MR do?

The documentation seems to be missing one step while exporting a project. Select **General** is not in the steps. A person will check the Advance menu in the setting menu, which is wrong.

The new menu should be as follows

1. On the top bar, select Menu > Projects and find your project.
2. On the left sidebar, select Settings.
3. Select General
4. Expand Advanced.
5. Select Export project.
6. After the export is generated, you should receive an email with a link to download the file.
7. Alternatively, you can come back to the project settings and download the file from there or generate a new export. After the file is available, the page should show the Download export button.

Related issues

https://gitlab.zendesk.com/agent/tickets/284430

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