[css-align-3] Make space-between's fallback safe, per WG resolution. w3c#10154

Author: tabatkins

css-align-3/Overview.bs

@@ -711,7 +711,7 @@ Distributed Alignment: the ''<content-distribution>/stretch'', ''space-between''
 				     alt="For example, given three items, all excess space is split in two and distributed: one half between the first two and one half between the last two items.">
 			</figure>
 
-			The default [=fallback alignment=] for this value is ''flex-start''.
+			The default [=fallback alignment=] for this value is ''safe flex-start''.
 			<span class='note'>(For layout modes other than flex layout, ''flex-start'' is identical to ''start''.)</span>
 
 		<dt><dfn>space-around</dfn>