[css-grid] Fix some markup errors.

Author: tabatkins

css-grid/Overview.bs

@@ -1088,7 +1088,7 @@ Grid Items</h2>
 
 		Note: This may distort the aspect ratio of the item, if it has one.
 	: all other values
-	:: Size the item as 'fit-content'.
+	:: Size the item as ''width/fit-content''.
 
 	<div class="note">
 		The following informative table summarizes the automatic sizing of grid items:
@@ -4235,8 +4235,8 @@ Changes</h2>
 			</blockquote>
 			<blockquote>
 			<ins>
-				<h3>
-				Stretch <css>auto</css> Tracks</h3>
+				<p>
+				Stretch <css>auto</css> Tracks</p>
 
 				<p>This step sizes expands tracks that have
 				an ''grid-template-columns/auto'' <a>max track sizing function</a>