[css-values] Fix bikeshed error. #1854

Author: fantasai

css-values-3/Overview.bs

@@ -691,7 +691,7 @@ Integers: the <<integer>> type</h3>
 	an <dfn export>integer</dfn> is one or more decimal digits ''0'' through ''9''
 	and corresponds to a subset of the <<number-token>> production
 	in the CSS Syntax Module [[!CSS3SYN]].
-	The first digit of an integer may be immediately preceded by ''-'' or ''+''
+	The first digit of an integer may be immediately preceded by <css>-</css> or <css>+</css>
 	to indicate the integer's sign.
 
 <!--

css-values-4/Overview.bs

@@ -680,7 +680,7 @@ Integers: the <<integer>> type</h3>
 	an <dfn export>integer</dfn> is one or more decimal digits ''0'' through ''9''
 	and corresponds to a subset of the <<number-token>> production
 	in the CSS Syntax Module [[!CSS3SYN]].
-	The first digit of an integer may be immediately preceded by ''-'' or ''+''
+	The first digit of an integer may be immediately preceded by <css>-</css> or <css>+</css>
 	to indicate the integer's sign.
 
 <!--