Fix a typo in 03-configuration.md

[hb326]

There was one little typo in the text which made the description of the example with "intake-esgf" unclear. This is the fix.

Before you start

• Read CONTRIBUTING.md.
• Create an issue to discuss your idea. This allows your contributions to be incorporated into the tutorial.

Tasks

• Give this pull request a descriptive title.
• If you are contributing to existing lesson materials, please make sure the content conforms to the Lesson development section in CONTRIBUTING.md and does not contain any spelling or grammatical errors.
• If you are making a new episode, please make sure the content conforms to the Lesson organization and Lesson formatting sections in CONTRIBUTING.md and does not contain any spelling or grammatical errors.
• Preferably Codacy checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why.
• Preview changes on your machine before pushing them to GitHub by running make serve, alternatively make docker-serve. Please see the Previewing your changes locally section in CONTRIBUTING.md for installation instructions.
• All code instructions have been tested.

If you need help with any of the tasks above, please do not hesitate to ask by commenting in the issue or pull request.

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Closes {Link to the corresponding issue}

[LisaBock]

Thanks @hb326