[css-contain-1] Fix grammar

Author: frivoal

css-contain-1/Overview.bs

@@ -563,7 +563,7 @@ Size Containment</h3>
 	are excluded,
 	because the table layout algorithm
 	does not allow boxes to become smaller than their inflow content.
-	Sizing a table cell as if it was empty and then layout out its content inside without changing the size
+	Sizing a table cell as if it was empty and then laying out its content inside without changing the size
 	is effectively an undefined operation.
 	Manually setting the 'width' or 'height' properties to ''0''
 	cannot make it smaller than its content.