dillon1000/react

Add a couple missing SVG tags to DOM Elements docs (#8383)

My first contribution to React!

While upgrading a React project, I found some suspect SVG that needed updating, so I dug in after checking the docs. I knew that support for some SVG properties had been added (namely `xmlns` and `xmlnsXlink`), but I noticed them missing from the reference's attribute list. This pull request updates `reference-dom-elements.md` by adding said properties.