[css-syntax-3] Fix typo in <an+b> value definition (w3c#7409)

Author: cdoublev

css-syntax-3/Overview.bs

@@ -2980,7 +2980,7 @@ The <code>&lt;an+b></code> type</h3>
 		  '+'?<sup><a href="#anb-plus">†</a></sup> n- <var>&lt;signless-integer></var> |
 		  -n- <var>&lt;signless-integer></var> |
 
-		  <var>&lt;n-dimension></var> ['+' | '-'] <var>&lt;signless-integer></var>
+		  <var>&lt;n-dimension></var> ['+' | '-'] <var>&lt;signless-integer></var> |
 		  '+'?<sup><a href="#anb-plus">†</a></sup> n ['+' | '-'] <var>&lt;signless-integer></var> |
 		  -n ['+' | '-'] <var>&lt;signless-integer></var>
 	</pre>