[css-grid] Fix confusing uses of 'layer'.

Author: fantasai

css-grid/Overview.bs

@@ -76,7 +76,7 @@ Introduction and Overview</h2>
 	Grid layout contains features targeted at web application authors.
 	The grid can be used to achieve many different layouts.
 	It excels at dividing up space for major regions of an application,
-	or defining the relationship in terms of size, position, and layer
+	or defining the relationship in terms of size, position, and painting layer
 	between parts of a control built from HTML primitives.
 
 	Like tables,
@@ -85,7 +85,7 @@ Introduction and Overview</h2>
 	grid layout doesn't have content structure,
 	and thus enables a wide variety of layouts not possible with tables.
 	For example, the children of a grid container can position themselves
-	such that they overlap and layer similar to positioned elements.
+	such that they overlap similar to positioned elements.
 
 	In addition, the absence of content structure in grid layout helps to manage changes to layout
 	by using fluid and source order independent layout techniques.