[css-grid-3][masonry] Add Brandon Stewart as an editor to Masonry Spec (#9548)

Author: stwrt

css-grid-3/Overview.bs

@@ -10,6 +10,7 @@ Editor: Tab Atkins Jr., Google, http://www.xanthir.com/contact/, w3cid 42199
 Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Mats Palmgren, Mozilla, mailto:mats@mozilla.com
 Editor: Jen Simmons, Apple, http://jensimmons.com/
+Editor: Brandon Stewart, Apple, https://brandonstewart.net
 Abstract: This module introduces masonry layout as an additional layout mode for <a href="https://www.w3.org/TR/css-grid-2/">CSS Grid</a> containers.
 WPT Path Prefix: css/css-grid/masonry/tentative/
 Markup Shorthands: css yes