[css-flexbox] Fix broken link.

Author: fantasai

css-flexbox/Overview.bs

@@ -2228,7 +2228,7 @@ Flex Container Baselines</h3>
 	a flex container provides a baseline just as a line box or table-row does. [[!CSS21]]
 
 	See [[css-writing-modes-3#intro-baselines]]
-	and [[css-align-3##baseline-rules]]
+	and [[css-align-3#baseline-rules]]
 	for more information on baselines.
 
 <!--