adding reference to CMIP6 CMOR tables

[rswamina]

Pull Request checklist

We appreciate your time and effort to improve the tutorial. Please keep in mind that lesson maintainers are volunteers and it may be some time before they can respond to your contribution.

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• Read CONTRIBUTING.md.
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Tasks

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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 {https://github.com/ESMValGroup/ESMValTool_Tutorial/issues/427}

[rswamina]

@LisaBock - I ahve now added a link to CMOR tables. If you are happy, can you approve changes.

[LisaBock]

Thanks @rswamina !

I just added a full stop at the end of the sentence.

[LisaBock]

Suggested change

	[CMIP6 CMOR Tables][cmip6-cmor-tables]{:target="_blank"}
	[CMIP6 CMOR Tables][cmip6-cmor-tables]{:target="_blank"}.