[css-position-3] Fix incorrect summary of flex static positioning w3c#7612

Author: fantasai

css-position-3/Overview.bs

@@ -837,7 +837,8 @@ Resolving Automatic Insets: the “Static-Position Rectangle”</h4>
 		<dt>Flex Layout
 		<dd>
 			The [=static-position rectangle=] of the child of a [=flex container=]
-			corresponds to the [=content edges=] of the [=flex container=].
+			corresponds to the [=content edges=] of the [=flex container=] in the [=cross axis=],
+			and to the [=outer edges=] of its hypothetical position in the [=main axis=].
 			See <a href="https://www.w3.org/TR/css-flexbox-1/#abspos-items">static position of a flex container child</a> in [[!CSS-FLEXBOX-1]].
 
 		<dt>Grid Layout