[css-values-3] Fix markup.

Author: fantasai

css-values-3/Overview.bs

@@ -1572,7 +1572,7 @@ Syntax</h4>
 	<dfn>&lt;calc-sum></dfn> = <<calc-product>> [ [ '+' | '-' ] <<calc-product>> ]*
 	<dfn>&lt;calc-product></dfn> = <<calc-value>> [ '*' <<calc-value>> | '/' <<calc-number-value>> ]*
 	<dfn>&lt;calc-value></dfn> = <<number>> | <<dimension>> | <<percentage>> | ( <<calc-sum>> )
-	dfn>&lt;calc-number-sum></dfn> = <<calc-number-product>> [ [ '+' | '-' ] <<calc-number-product>> ]*
+	<dfn>&lt;calc-number-sum></dfn> = <<calc-number-product>> [ [ '+' | '-' ] <<calc-number-product>> ]*
 	<dfn>&lt;calc-number-product></dfn> = <<calc-number-value>> [ '*' <<calc-number-value>> | '/' <<calc-number-value>> ]*
 	<dfn>&lt;calc-number-value></dfn> = <<number>> | ( <<calc-number-sum>> )
 	</pre>