[css-overflow] Typo fix (#12621)

Author: andreubotella

css-overflow-4/Overview.bs

@@ -713,7 +713,7 @@ Indicating Block-Axis Overflow: the 'block-ellipsis' property</h3>
 	but as it inherits, will have an effect on descendants’ line boxes unless overridden.
 
 	This property only affects a line box
-	if it immediately precedes eiher a [=region break=]
+	if it immediately precedes either a [=region break=]
 	or a [=clamp point=] in the [=block formatting context=].
 
 	Note: See [[css-overflow-4#fragmentation]] for a way to generate boxes with such a [=region break=].