[css-images-4] Fix grammar and railroad diagram for color stop hints (related: #976)

Author: LeaVerou

css-images-4/Overview.bs

@@ -821,13 +821,13 @@ Gradient Color-Stops</h3>
 
 	<pre class=prod>
 		<dfn>&lt;color-stop-list></dfn> =
-			[ <<linear-color-stop>> , <<linear-color-hint>>? ]# , <<linear-color-stop>>
+			[ <<linear-color-stop>> [, <<linear-color-hint>>]? ]# , <<linear-color-stop>>
 		<dfn>&lt;linear-color-stop></dfn> = <<color>> && <<color-stop-length>>
 		<dfn>&lt;linear-color-hint></dfn> = <<length-percentage>>
 		<dfn>&lt;color-stop-length></dfn> = <<length-percentage>>{1,2}
 
 		<dfn>&lt;angular-color-stop-list></dfn> =
-			[ <<angular-color-stop>> , <<angular-color-hint>>? ]# , <<angular-color-stop>>
+			[ <<angular-color-stop>> [, <<angular-color-hint>>]? ]# , <<angular-color-stop>>
 		<dfn>&lt;angular-color-stop></dfn> = <<color>> && <<color-stop-angle>>?
 		<dfn>&lt;angular-color-hint></dfn> = <<angle-percentage>>
 		<dfn>&lt;color-stop-angle></dfn> = <<angle-percentage>>{1,2}
@@ -839,11 +839,13 @@ Gradient Color-Stops</h3>
 		Plus:
 			Sequence:
 				N: <color-stop>
-				T: ,
 				Choice: 1
-					N: <color-hint>
+					Sequence:
+						T: ,
+						N: <color-hint>
 					Skip:
 			T: ,
+		T: ,
 		N: <color-stop>
 	</pre>