[css-color-4] fix w3c#4648 gray is percentage

Author: svgeesus

css-color-4/Overview.bs

@@ -1940,19 +1940,19 @@ 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'>
-	<dfn>gray()</dfn> = gray( <<number>>  [ / <<alpha-value>> ]? )
+	<dfn>gray()</dfn> = gray( <<percentage>>  [ / <<alpha-value>> ]? )
 	</pre>
 
 	The first argument specifies the shade of gray,
 	equal to the CIE Lightness,
 	while the second optional argument specifies the alpha channel of the gray.
 
 	Note: In other words,
-	''gray(a / b)'' is equal to ''lab(a 0 0 / b)''.
+	''gray(a% / b)'' is equal to ''lab(a% 0 0 / b)''.
 
 <h3 id='gray-to-rgb'>
 Converting gray colors to sRGB  colors</h3>