[css-text-4] typo letter->character

Author: fantasai

css-text-4/Overview.bs

@@ -10261,7 +10261,7 @@ Line Start/End Padding: the 'line-padding' property</h3>
 	</pre>
 
 	Whereas 'letter-spacing' adjusts spacing
-	between <a>typographic letter units</a>
+	between <a>typographic character units</a>
 	and does not apply at the start or end of a line,
 	this property adjusts spacing only at the start/end of a line.
 	The extra spacing is applied only