[css-color-4] correct gray() example

Author: jonathantneal

css-color-4/Overview.bs

@@ -1752,7 +1752,7 @@ Specifying Grays: the ''gray()'' functional notation</h2>
 	Grays are fully desaturated colors.
 	The ''gray()'' functional notation simplifies specifying this common set of colors,
 	so that only a single numerical parameter is required,
-	and so that gray(50%) is a visual mid-gray
+	and so that gray(50) is a visual mid-gray
 	(perceptually equidistant between black and white).
 
 	<pre class='prod'>