[css-values-4] Don't dfn 'string'; instead refer to Infra.

Author: tabatkins

css-values-4/Overview.bs

@@ -25,6 +25,7 @@ spec: css-sizing-3; type: property;
 	text: min-width
 	text: box-sizing
 spec: selectors-4; type: type; text: <wq-name>
+spec: infra; type: dfn; text: string;
 </pre>
 <style>
 code, small { white-space: nowrap }
@@ -571,7 +572,7 @@ Author-defined Identifiers: the <<custom-ident>> type</h3>
 <h3 id="strings">
 Quoted Strings: the <<string>> type</h3>
 
-	<dfn export lt="string">Strings</dfn> are denoted by <dfn id="string-value">&lt;string></dfn>
+	[=Strings=] are denoted by <dfn id="string-value">&lt;string></dfn>
 	and consist of a sequence of characters delimited by double quotes or
 	single quotes. They correspond to the <<string-token>> production
 	in the <a href="https://www.w3.org/TR/css-syntax/">CSS Syntax Module</a> [[!CSS3SYN]].