[css-flexbox-1] Fix fallback alignment for align-content:stretch

The commit b6c01a4 meant to edit only `space-*`
values. Also, CSS Align spec has `align-content:stretch` fallback value as
`flex-start` per
https://drafts.csswg.org/css-align-3/#valdef-align-content-stretch

Author: aethanyc

css-flexbox-1/Overview.bs

@@ -2249,7 +2249,7 @@ Packing Flex Lines: the 'align-content' property</h3>
 		<dd>
 			Lines stretch to take up the remaining space.
 			If the leftover free-space is negative,
-			this value falls back to ''safe flex-start''.
+			this value falls back to ''flex-start''.
 			Otherwise,
 			the free-space is split equally between all of the lines,
 			increasing their cross size.