[css-values-4] Rephrase example to use px, so there's no distracting computed-value unit change when it's not the point of the example. Fixes #3806.

Author: tabatkins

css-values-4/Overview.bs

@@ -3086,8 +3086,8 @@ Computed Value</h3>
 
 	Where percentages are not resolved at computed-value time,
 	they are not resolved in [=math functions=],
-	e.g. ''calc(100% - 100% + 1em)'' resolves to ''calc(1em + 0%)'',
-	not to ''1em''.
+	e.g. ''calc(100% - 100% + 1px)'' resolves to ''calc(1px + 0%)'',
+	not to ''1px''.
 	If there are special rules for computing percentages in a value
 	(e.g. <a href="https://www.w3.org/TR/CSS21/visudet.html#the-height-property">the <css>height</css> property</a>),
 	they apply whenever a [=math function=] contains percentages.