[css-flexbox-1] Fix markup error

Author: fantasai

css-flexbox-1/Overview.bs

@@ -2884,8 +2884,8 @@ Flex Container Intrinsic Main Sizes</h4>
 	its [[#intrinsic-item-contributions|max-content contribution]],
 	to the extent allowed by the items’ flexibility.
 
-	The <strong>[=min-content=] [=main size=] is, theoretically,
-	the smallest size the [=flex container can take=]
+	The <strong>[=min-content=] [=main size=]  of a <a>flex container</a></strong>
+	is, theoretically, the smallest size the [=flex container can take=]
 	such that no items overflow it,
 	and no item's contents overflow the item--
 	setting aside the cases in which the boxes layouts are <em>defined</em> to overflow