typo fix

<string> was considered a tag, so switched to <dfn><<string>></dfn>  so it would be visible

Author: estelle

css-shapes-2/Overview.bs

@@ -142,7 +142,7 @@ Supported Shapes</h3>
 					or ''evenodd''.
 					Default value when omitted is ''nonzero''.</li>
 				<li>
-					The <string> represents an
+					The <dfn><<string>></dfn>  represents an
 					<a href="https://www.w3.org/TR/SVG11/paths.html#PathData">SVG Path data string</a>.
 					The path data string must be conform
 					to the grammar and parsing rules of SVG 1.1.