Fix incorrect references in markerUnits docs

[atdyer]

The markerUnits attribute defines the coordinate system for the markerWidth and markerHeight attributes, not markerWidth and markerUnits as is currently reflected in the docs. This commit changes the docs to match the SVG 1.1 and SVG 2 specs.

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[sideshowbarker]

Tristan, thanks much for catching this and fixing it, and congrats on landing your first docs change here — welcome aboard 🎉