[css-align] Fix markup of the 'stretch' keyword.

Author: tabatkins

css-align/Overview.bs

@@ -639,7 +639,7 @@ Inline/Main-Axis Alignment: the 'justify-self' property</h3>
 
 	When the box’s computed 'width'/'height' (as appropriate to the axis) is ''width/auto''
 	and neither of its margins (in the appropriate axis) are ''margin/auto'',
-	the <dfn for="justify-self, align-self">stretch</dfn> keyword
+	the <dfn value for="justify-self, align-self">stretch</dfn> keyword
 	sets the box’s used size to the length necessary to make its outer size
 	as close to filling the <a>alignment container</a> as possible
 	while still respecting the constraints imposed by 'min-height'/'min-width'/'max-height'/'max-width'.
@@ -751,7 +751,7 @@ Block/Cross-Axis Alignment: the 'align-self' property</h3>
 	(minus any ''legacy'' keywords) on all other boxes,
 	or ''align-self/start'' if the box has no parent.
 	Its behavior depends on the layout model, as described for 'justify-self'.
-	
+
 	The ''align-self/stretch'' keyword is as defined in [[#justify-self-property]].
 
 	All other values are as defined in [[#alignment-values]], above.