[css-display-4] Update reading flow definition

Author: rachelandrew

css-display-4/Overview.bs

@@ -1170,7 +1170,7 @@ A <dfn export>reading flow container</dfn> is a block, flex or grid container
 with a valid 'reading-flow' value other than ''normal''.
 
 The <dfn export>rendering-defined sibling reading flow</dfn> of a <a>reading flow container</a>
-is an ordered list of in-flow children of the container.
+is an ordered list of the children of the container.
 All children must render to an element and are considered siblings in the reading flow container.
 The order is determined by the 'reading-flow' property.