Fix typo (#12686)

Author: Psychpsyo

css-align-3/Overview.bs

@@ -2064,7 +2064,7 @@ Row and Column Gutters: the 'row-gap' and 'column-gap' properties</h3>
 			see [[#gap-percent]].
 
 		: <dfn>normal</dfn>
-		:: The ''gap/normal'' represents
+		:: The value ''gap/normal'' represents
 			a used value of ''1em'' on <a>multi-column containers</a>,
 			and a used value of ''0px'' in all other contexts.
 	</dl>