[css-flexbox] Update DoC, add Greg to Acks.

Author: tabatkins

css-flexbox/Overview.bs

@@ -2782,6 +2782,7 @@ Morten Stenshorne,
 Christian Stockwell,
 Ojan Vafai,
 Eugene Veselov,
+Greg Whitworth,
 Boris Zbarsky.
 
 <h2 class="no-num" id="changes">Changes</h2>
@@ -2913,7 +2914,7 @@ Changes since the 18 September 2012 Candidate Recommendation</h3>
 			<blockquote>
 				<ins>
 				<!--
-					<h3 id='intrinsic-sizes'>
+					<h3 id="intrinsic-sizes">
 					Intrinsic Sizes</h3>
 				-->
 					<p>