[css-flexbox-1] Clean up wording for dividing flex factor in finding …

…max-content flex fraction. Re-applies fix for w3c#1803, which was lost during revert of w3c#1735.

css-flexbox-1/Overview.bs

@@ -2790,8 +2790,11 @@ Flex Container Intrinsic Main Sizes</h4>
 		<li>
 			For each <a>flex item</a>,
 			subtract its outer <a>flex base size</a> from its [[#intrinsic-item-contributions|max-content contribution]] size.
-			If that result is not zero, divide it by (if the result was positive) its <a>flex grow factor</a> floored at 1,
-			or (if the result was negative) by its <a>scaled flex shrink factor</a>, having floored the <a>flex shrink factor</a> at 1.
+			If that result is positive,
+			divide by its <a>flex grow factor</a> floored at 1;
+			if negative,
+			divide by its <a>scaled flex shrink factor</a>
+			having floored the <a>flex shrink factor</a> at 1.
 			This is the item's <var>max-content flex fraction</var>.
 
 		<li>