[css-module-bikeshed] Fix duplicate IDs.

Author: dbaron

css-module-bikeshed/Overview.bs

@@ -64,7 +64,7 @@ Sample section {#sample-topic}
 
 	Note: Note that the property will automatically be linked to its definition.
 
-	To define <dfn>terms</dfn> into the <dfn>index</dfn>,
+	To define <dfn>terms</dfn> into the <dfn id="dfn-index">index</dfn>,
 	there are many <dfn title="variant">variants</dfn>,
 	but hopefully the <dfn title="simple|simpler|simplest">simplest</dfn>
 	will be the most common.