[css-values] Fix ID

Author: fantasai

css-values/Overview.bs

@@ -678,7 +678,7 @@ Relative lengths</h3>
 Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 
 	Aside from ''rem'' (which refers to the font-size of the root element),
-	the <dfn export id="font-relative length">font-relative lengths</dfn> refer to the font metrics of the element on which they are used.
+	the <dfn export id="font-relative-length">font-relative lengths</dfn> refer to the font metrics of the element on which they are used.
 	The exception is when they occur in the value of the 'font-size' property itself,
 	in which case they refer to the computed font metrics of the parent element
 	(or the computed font metrics corresponding to the initial values of the 'font' property,