Editorial: Fix term reference

Author: dirkschulze

css-masking-1/Overview.bs

@@ -49,6 +49,7 @@ spec:filter-effects-1; type:property;
 spec:filter-effects-1; type:element;
     text:fecolormatrix
     text:filter
+spec:css-transforms-1; type:dfn; text:user coordinate system
 </pre>
 <pre class=anchors>
 spec:svg2; url:https://svgwg.org/svg2-draft/coords.html#ViewBoxAttribute; type:element-attr; for:svg; text:viewBox