Update 14-looping-data-sets.md

[wgriffa]

This avoids the FutureWarning: The default value of numeric_only in DataFrame.mean is deprecated. In a future version, it will default to False. In addition, specifying 'numeric_only=None' is deprecated. Select only valid columns or specify the value of numeric_only to silence this warning.

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⏱️ Updated at 2023-10-17 11:53:55 +0000

[alee]

argh! somehow I missed this in the conversation about #670 - apologies for that and thank you for raising this, I should have looked through the earlier pull requests.

Thank you for submitting this, closing as this is now addressed in #674