[cssom-view] Typo scrollX -> scrollY

Fixes http://www.w3.org/mid/CAEfS-pjUd41OLs4eprpkGrdAk6q-x9eaBbqknmDm-zJ4g7zuYQ@mail.gmail.com.

Author: zcorpan

cssom-view/Overview.bs

@@ -1449,7 +1449,7 @@ The <dfn attribute for=MouseEvent>pageX</dfn> attribute must follow these steps:
 The <dfn attribute for=MouseEvent>pageY</dfn> attribute must follow these steps:
 
 1. If the event's <a>dispatch flag</a> is set, return the vertical coordinate of the position where the event occurred relative to the origin of the <a>initial containing block</a> and terminate these steps.
-1. Let <var>offset</var> be the value of the {{Window/scrollX}} attribute of the event's associated {{Window}} object, if there is one, or zero otherwise.
+1. Let <var>offset</var> be the value of the {{Window/scrollY}} attribute of the event's associated {{Window}} object, if there is one, or zero otherwise.
 1. Return the sum of <var>offset</var> and the value of the event's {{MouseEvent/clientY}} attribute.
 
 The <dfn attribute for=MouseEvent>clientX</dfn> attribute must return the x-coordinate of
@@ -1694,7 +1694,9 @@ Stewart Brodie,
 Sylvain Galineau,
 Tab Atkins,
 Tarquin Wilton-Jones,
-Thomas Moore, and
+Thomas Moore,
+Thomas Shinnick,
+and
 Xiaomei Ji
 for their contributions to this document.