[css-fonts-4] dedupe font-synthesis IDs on intro and shorthand

Author: svgeesus

css-fonts-4/Overview.bs

@@ -1294,7 +1294,7 @@ Shorthand font property: the 'font' property</h3>
 		</pre>
 	</div>
 
-<h3 id="font-synthesis">
+<h3 id="font-synthesis-intro">
 Controlling synthetic faces</h3>
 
 <h4 id="font-synthesis-weight">