[css-grid] Fix markup to point to correct 'stretch' keywords.

Author: tabatkins

css-grid/Overview.bs

@@ -2917,7 +2917,7 @@ Alignment and Spacing</h2>
 
 	By default,
 	<a>grid items</a> stretch to fill their <a>grid area</a>.
-	However, if 'justify-self' or 'align-self' compute to a value other than ''stretch''
+	However, if 'justify-self' or 'align-self' compute to a value other than ''justify-self/stretch''
 	or margins are <a value for="margin">auto</a>,
 	<a>grid items</a> will auto-size to fit their contents.
 
@@ -3051,7 +3051,7 @@ Aligning the Grid: the 'justify-content' and 'align-content' properties</h3>
 	Note that certain values of 'justify-content' and 'align-content'
 	can cause the tracks to be spaced apart
 	(''justify-content/space-around'', ''justify-content/space-between'', ''justify-content/space-evenly'')
-	or to be resized (''stretch'').
+	or to be resized (''&lt;content-distribution>/stretch'').
 
 	Issue: Add example of fixed size spanner getting extra space from content-distribution.