[css-ruby] Don't be overly-specific on the type of principal box.

Author: fantasai

css-ruby-1/Overview.bs

@@ -222,7 +222,7 @@ Non-Inline Ruby</h4>
 	<p>If an element has an <i>inner display type</i> of ''ruby''
 	and an <i>outer display type</i> other than ''display/inline'',
 	then it generates two boxes:
-	a principal block container box of the required <i>outer display type</i> type,
+	a principal box of the required <i>outer display type</i> type,
 	and an inline-level <i>ruby container</i>.
 	All properties specified on the element apply to the principal box
 	(and if inheritable, inherit to the <i>ruby container box</i>).