Fix typo (<var> wanted to be </var)

This fixes a typo in @fantasai's commit w3c@1ddd303

(Without this typo-fix, a bunch of this paragraph ends up being incorrectly italicized.)

Author: dholbert

css-flexbox/Overview.bs

@@ -2798,7 +2798,7 @@ Flex Container Intrinsic Main Sizes</h4>
 
 		<li>
 			For each <a>flex item</a>,
-			find its <var>max-content flex fraction<var>:
+			find its <var>max-content flex fraction</var>:
 			Subtract its outer <a>flex base size</a> from its [[#intrinsic-item-contributions|max-content contribution]] size.
 			If that result is positive,
 			divide by its <a>flex grow factor</a> floored at 1;