[css-flexbox] Clarify that we're using the *outer* flex base size.

Author: fantasai

css-flexbox/Overview.bs

@@ -2655,7 +2655,7 @@ Intrinsic Sizes</h3>
 	<ol>
 		<li>
 			For each <a>flex item</a>,
-			subtract its <a>flex base size</a> from its <a>max-content contribution</a> size.
+			subtract its outer <a>flex base size</a> from its <a>max-content contribution</a> size.
 			If that result is not zero, divide it by its <a>flex grow factor</a> (if the result was positive)
 			or by its <a>scaled flex shrink factor</a> (if the result was negative).
 			This is the item's <var>max-content flex fraction</var>.