Q: Help! EPUB Validator Error: "The property 'svg' should be declared in the OPF file"

Hello, newbie ePub creator here. So I created a print book in InDesign, the re-formatted the book and exported as an ePub 3.0. Then I did some fine tuning of the code using Sigil. When running my book through the ePub Validator for iBooks (http://validator.idpf.org/) I have one error I just can't seem to solve. It is:

"The property 'svg' should be declared in the OPF file"

The error message refers to the cover file of my book as the problem file. I actually replaced the cover in Sigil, and I'm not sure if this is part of the issue. I am attaching the error message, as well as screenshots of my "cover.xhtml" and "content.opf" file, which I think are where the issue lies.

Can anyone help?