[css-flexbox] Fix broken link.

Author: tabatkins

css-flexbox/Overview.bs

@@ -1146,7 +1146,7 @@ Reordering and Accessibility</h4>
 
 		<p>
 			As an added bonus,
-			the columns will all be <a href="#propdef-align-stretch">equal-height</a> by default,
+			the columns will all be <a href="#valuedef-stretch0">equal-height</a> by default,
 			and the main content will be as wide as necessary to fill the screen.
 			Additionally,
 			this can then be combined with media queries to switch to an all-vertical layout on narrow screens:

css-flexbox/Overview.html

@@ -1204,7 +1204,7 @@ <h4 class="heading settled heading" data-level=5.4.1 id=order-accessibility><spa
 #main &gt; aside   { order: 3; width: 200px; }</pre>
 		<p>
 			As an added bonus,
-			the columns will all be <a href=#propdef-align-stretch>equal-height</a> by default,
+			the columns will all be <a href=#valuedef-stretch0>equal-height</a> by default,
 			and the main content will be as wide as necessary to fill the screen.
 			Additionally,
 			this can then be combined with media queries to switch to an all-vertical layout on narrow screens: