[css-flexbox-1] Fix a broken link

Author: xfq

css-flexbox-1/Overview.bs

@@ -3874,7 +3874,7 @@ Substantive Changes and Bugfixes</h4>
 			</blockquote>
 		<li id="change-2016-align-self-as-specified">
 			For ease of implementation, ''align-self/auto'' value of 'align-self' now computes to itself always.
-			See <a href="change-2015-align-self-auto">related previous change</a>
+			See <a href="#change-2015-align-self-auto">related previous change</a>
 			requiring this computation for absolutely-positioned elements.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/440">Issue 440</a>,
 			<a href="https://github.com/w3c/csswg-drafts/issues/644">Issue 644</a>)