[css-grid] Rename heading.

--HG--
extra : rebase_source : 8175163b7acfdf540b0115426e85b52c926aff7f

Author: tabatkins

css-grid/Overview.bs

@@ -192,7 +192,7 @@ Adapting Layouts to Available Space</h3>
 	</pre>
 
 <h3 id='source-independence'>
-Source Independence</h3>
+Source-Order Independence</h3>
 
 	<figure class="sidefigure">
 		<img alt="Image: An arrangement suitable for portrait orientation." src="images/game-portrait.png" />